First published Sun Aug 30, 2015

This entry discusses philosophical idealism as a movement chiefly in the eighteenth and nineteenth centuries, although anticipated by certain aspects of seventeenth century philosophy. It examines the relationship between epistemological idealism (the view that the contents of human knowledge are ineluctably determined by the structure of human thought) and ontological idealism (the view that epistemological idealism delivers truth because reality itself is a form of thought and human thought participates in it). After discussing precursors, the entry focuses on the eighteenth-century versions of idealism due to Berkeley, Hume, and Kant, the nineteenth-century movements of German idealism and subsequently British and American idealism, and then concludes with an examination of the attack upon idealism by Moore and Russell.

1. Introduction

The terms “idealism” and “idealist” are by no means used only within philosophy; they are used in many everyday contexts as well. Optimists who believe that, in the long run, good will prevail are often called “idealists”. This is not because such people are thought to be devoted to a philosophical doctrine but because of their outlook on life generally; indeed, they may even be pitied, or perhaps envied, for displaying a naïve worldview and not being philosophically critical at all. Even within philosophy, the terms “idealism” and “idealist” are used in different ways, which often makes their meaning dependent on the context. However, independently of context one can distinguish between a descriptive (or classificatory) use of these terms and a polemical one, although sometimes these different uses occur together. Their descriptive use is best documented by paying attention to the large number of different “idealisms” that appear in philosophical textbooks and encyclopedias, ranging from metaphysical idealism through epistemological and aesthetic to moral or ethical idealism. Within these idealisms one can find further distinctions, such as those between subjective, objective and absolute idealism, and even more obscure characterizations such as speculative idealism and transcendental idealism. It is also remarkable that the term “idealism”, at least within philosophy, is often used in such a way that it gets its meaning through what is taken to be its opposite: as the meaningful use of the term “outside” depends on a contrast with something considered to be inside, so the meaning of the term “idealism” is often fixed by what is taken to be its opposite. Thus, an idealist is someone who is not a realist, not a materialist, not a dogmatist, not an empiricist, and so on. Given the fact that many also want to distinguish between realism, materialism, dogmatism, and empiricism, it is obvious that thinking of the meaning of “idealism” as determined by what it is meant to be opposed to leads to further complexity and gives rise to the impression that underlying such characterizations lies some polemical intent.

It nevertheless seems safe to say that within modern philosophy there have been two fundamental conceptions of idealism:

something mental (the mind, spirit, reason, will) is the ultimate foundation of all reality, or even exhaustive of reality, and although the existence of something independent of the mind is conceded, everything that we can know about this mind-independent “reality” is held to be so permeated by the creative, formative, or constructive activities of the mind (of some kind or other) that all claims to knowledge must be considered, in some sense, to be a form of self-knowledge.

Idealism in sense (1) may be called “metaphysical” or “ontological idealism”, while idealism in sense (2) may be called “formal” or “epistemological idealism”. The modern paradigm of idealism in sense (1) might be considered to be George Berkeley’s “immaterialism”, according to which all that exists are ideas and the minds, less than divine or divine, that have them. (Berkeley himself did not use the term “idealism”.) The fountainhead for idealism in sense (2) might be the position that Immanuel Kant asserted (if not clearly in the first edition of his Critique of Pure Reason (1781) then in his Prolegomena to Any Future Metaphysics (1783) and in the “Refutation of Idealism” in the second edition of the Critique) according to which idealism does “not concern the existence of things”, but asserts only that our “modes of representation” of them, above all space and time, are not “determinations that belong to things in themselves” but features of our own minds. Kant called his position “transcendental” and “critical” idealism, and it has also been called “formal” idealism. In the interest of generality, we will call this sort of position “epistemological idealism”. Because of the many ways in which the term “metaphysical” is used—for example, Kant himself used the term “metaphysics” in two different senses, in one sense to refer to the derivation of knowledge from pure reason that he discredited but in another to refer to his own theory of the sources of knowledge, what we now call his epistemology—we will call idealism in sense (1) “ontological idealism”. If one accepts this characterization, then ontological idealism is meant to be opposed to both dualism, according to which reality ultimately consists not just of mental “stuff” but also of mind-independent matter, and to materialism, which takes matter to be all there is, while epistemological idealism is opposed to materialism but not necessarily to dualism. Epistemological idealism is sometimes motivated by the simple thought that whatever we know, we must know from our own perspective, but is sometimes motivated by further arguments. It does not automatically imply ontological idealism without further assumptions, although a commitment to ontological idealism obviously includes commitment to epistemological idealism since, assuming it allows for the possibility of knowledge at all, it allows nothing but the mental to be known. The further assumptions that lead from epistemological to ontological idealism can be a simple desire to avoid the possibility of doubt or ignorance by collapsing the distinction between knowledge and what is known, as when Berkeley claims that only his immaterialism can defend common sense, but can take other forms as well.

In what follows, we will concentrate mainly on the discussion of philosophical theories of idealism rather than the popular, everyday sense of the term. It is worth noting, however, that in its complex history—above all in the social as well as philosophical movement that dominated British and American universities in the second half of the nineteenth century and through the first World War—idealism in either of its philosophical forms was indeed connected to idealism in the popular sense of progressive and optimistic social thought.

The distinction between epistemological and ontological idealism that we are making here is hardly novel, although it was not made by many of the seventeenth- and eighteenth-century philosophers to be discussed below. The distinction was clearly formulated only in the nineteenth century. For example, the American philosopher Josiah Royce made this distinction at the end of the nineteenth century (albeit without our concern for the many possible meanings of “metaphysics”) when he called what we are terming ontological idealism “metaphysical” idealism. On his straightforward definitions, epistemological idealism

involves a theory of the nature of our human knowledge; and various decidedly different theories are called by this name in view of one common feature, namely, the stress that they lay upon the ‘subjectivity’ of a larger or smaller portion of what pretends to be our knowledge of things

Metaphysical idealism, he says, “is a theory as to the nature of the real world, however we may come to know that nature”, namely, as he says quoting from another philosopher of the time, “‘the belief in a spiritual principle at the basis of the world, without the reduction of the physical world to a mere illusion’” (Royce, The Spirit of Modern Philosophy, pp. xii–xiii). Royce’s own view was that epistemological idealism ultimately works only on a foundation of metaphysical idealism, in particular that “the question as to how we ‘transcend’ the ‘subjective’ in our knowledge”, that is, the purely individual, although it exists for both metaphysical realists and idealists, can only be answered by metaphysical idealists (p. xiv). We will suggest, on the contrary, that while there are many good reasons for epistemological idealism, indeed, that—suitably broadly understood—it has in fact become the default epistemology of modern philosophy, many of the most important of modern idealists have sought to avoid any inference from epistemological to ontological idealism. This was particularly true in the twentieth century, when tendencies toward epistemological idealism were in fact widespread in many schools of philosophy (although for different reasons than in the eighteenth century, reasons to be touched upon in the final section of this entry), very few philosophers were willing to identify themselves as idealists, even merely epistemological idealists.

As always when philosophy must decide between alternatives, there must be reasons or motives for deciding one way or the other. Since philosophical idealism in either of its forms does not seem to be the most obvious way in which to understand the nature of reality and the conditions under which its constitution can be known, it is of interest to look into the reasons and motives for idealism. Here one can distinguish between two major kinds of motives: those which are grounded in self-conceptions, i.e., in convictions about the role that the self or the human being plays in the world, and those based on what might correspondingly be called world-convictions, i.e., on conceptions about the way the world is constituted objectively or at least appears to be constituted to a human subject. Concerning motives based on self-conceptions of human beings, idealism has seemed hard to avoid by many who have taken freedom in one of its many guises (freedom of choice, freedom of the will, freedom as autonomy) to be an integral part of any conception of the self worth pursuing, because the belief in the reality of freedom often goes together with a commitment to some version of mental causation, and it is tempting to think that the easiest (or at least the most economical) way to account for mental causation consists in “mentalizing” or idealizing all of reality, thus leading to ontological idealism, or at least to maintain that the kind of causal determinism that seems to conflict with freedom is only one of our ways of representing the world, thus leading to epistemological idealism. Motives for idealism based on world-convictions can be found in many different attitudes towards objectivity. If one is to believe in science as the best and only way to get an objective (subject-independent) conception of reality, one might still turn to idealism, at least epistemological idealism, because of the conditions supposed to be necessary in order to make sense of the very concept of a law (of nature) or of the normativity of logical inferences for nature itself. If one believes in the non-conventional reality of normative facts one might also be drawn to idealism in order to account for their non-physical reality—Plato’s idealism, which asserts the reality of non-physical Ideas to explain the status of norms and then reduces all other reality to mere simulacra of the former might be considered a forerunner of ontological idealism motivated by concern for the reality of norms. An inclination toward idealism might even arise from considerations pertaining to the ontological status of aesthetic values (is beauty an objective attribute of objects?) or from the inability or the unwillingness to think of the constitution of social and cultural phenomena like society or religion in terms of physical theory. In short: There are about as many motives and reasons for endorsing idealism as there are different aspects of reality to be known or explained.

Although we have just referred to Plato, the term “idealism” became the name for a whole family of positions in philosophy only in the course of the eighteenth century. Even then, those whom critics called “idealists” did not identify themselves as such until the time of Kant, and no sooner did the label come into use than did those to whom it was applied or who used it themselves attempt to escape it or refine it. As already mentioned, Berkeley, the paradigmatic ontological idealist in the British tradition, did not use the name for his own position, and Leibniz, at least some versions of whose monadology might be considered idealist, also did not call his position by that name. Rather, in contrasting Epicurus with Plato, Leibniz called the latter an idealist and the former a materialist, because according to him idealists like Plato hold that “everything occurs in the soul as if there were no body” whereas on the materialism of Epicurus “everything occurs in the body as if there were no soul” (“Reply to the Thoughts on the System of Preestablished Harmony contained in the Second Edition of Mr. Bayle’s Critical Dictionary, Article Rorarius”, 1702, PPL, p. 578), although in this text Leibniz also says that his own view combines both of these positions. It seems to have been Christian Wolff who first used “idealism” explicitly as a classificatory term. Wolff, often considered the most dedicated Leibnizian of his time (although in fact his position was more eclectic than at least some versions of Leibniz’s) set out to integrate the terms “idealism” and “materialism” into his taxonomy of philosophical attitudes of those “who strive towards the knowledge and philosophy of things” in the Preface to the other [second] Edition of his so-called German Metaphysics [Vernünfftige Gedancken von Gott, der Welt und der Seele des Menschen, auch allen Dingen überhaupt, den Liebhabern der Wahrheit mitgetheilet (Halle: Carl Hermann Hemmerde, 1747)]. Wolff distinguishes between two basic attitudes, one of which he sees exemplified by the skeptic, the other by what he calls “the dogmatist”. The skeptic doubts the possibility of knowledge in general and thus refuses to defend any positive claim at all. By contrast, the dogmatist puts forward positive doctrines, and these can be divided into those which posit as fundamental either one single kind of entities [Art der Dinge] or two different kinds. Wolff names the supporters of the first position “monists” and the adherents of the second “dualists”. This amounts to the division of all dogmatic doctrines, i.e., all knowledge-claims with respect to the ultimate constitution of reality, into monistic and dualistic theories. Here is where the term “idealist” then makes its appearance in Wolff’s typology: he distinguishes within the monists between idealists and materialists. Idealists “concede only spirits or else those things that do not consist of matter”, whereas materialists “do not accept anything in philosophy other than the corporeal and take spirits and souls to be a corporeal force”. Dualists, on the contrary, are happy “to accept both bodies and spirits as real and mutually independent things”. Wolff then goes on to distinguish within idealism between “egoism” and “pluralism”, depending on whether an idealist thinks just of himself as a real entity or whether he will allow for more than one (spiritual) entity; the first of these positions would also come to be called solipsism, so that solipsism would be a variety of (ontological) idealism but not all idealism would be solipsism.

Wolff’s way of classifying a philosophical system was enormously influential in eighteenth-century Continental philosophy—for example, it was closely followed by Alexander Gottlieb Baumgarten in his 1739 Metaphysica, which was in turn used by Kant as the textbook for his metaphysics (and anthropology) lectures throughout his career, and whose definition of ontological (or “dogmatic”) idealism, as contrasted to his own “transcendental” or “critical” idealism, would also be that it is the position according to which there are only minds—and so it is no surprise that almost all talk about idealism was heavily influenced by Wolff’s characterization, at least as far ontological idealism was concerned. This is so because it reflects the main metaphysical disputes in seventeenth- and early eighteenth-century philosophy on the Continent quite well. In terms of Wolff’s distinctions, these disputes can be framed as disputes between (a) monists and dualists and (b) idealists and materialists; positions in this debate were often influenced by perplexities surrounding the (ontological) question of the interaction of substances, although they were also influenced by the (epistemological) debate over innatism. Although neither dualism, whose main representative was Descartes (who asserted the existence of both res cogitans and res extensa), nor monism, allegedly represented paradigmatically by Spinoza in its materialistic version (substantia, deus, natura) and by Leibniz in its idealistic form (monad, entelechy, simple substance) succeeded in finding satisfying answers to this and related questions, in the early modern era these disputes shaped the conception of what the object of metaphysics (metaphysica generalis sive ontologia) was supposed to be.

2. Idealism in Early Modern Rationalism

Prior to Wolff, neither defending nor refuting (ontological) idealism seems to have been a central issue for rationalist philosophers, and none of them called themselves idealists. Yet what are by later lights idealistic tendencies can nevertheless be found among them.

While Descartes’s “first philosophy” clearly defends dualism, he takes his target to be skepticism rather than idealism, and thus is from our point of view concerned to resist epistemological idealism; Spinoza defends a form of materialism, but takes his primary target to be pluralism as contrasted to monism; and Leibniz does not seem overly worried about choosing between ontologically idealist and dualist forms of his “monadology”, while his famous thesis that each monad represents the entire universe from its own point of view might be taken to be a form of epistemological idealism, but Leibniz does not seem to conceive of it as such. Nicolas Malebranche’s theory of “seeing all things in God” might be the closest we find to an explicit assertion of both epistemological and ontological idealism in seventeenth-century rationalism, and thus as a forerunner of the “absolute” idealism of the nineteenth century. While from a later point of view it may seem surprising that these rationalists were not more concerned with explicitly asserting or refuting one or both versions of idealism, perhaps they were more concerned with theological puzzles about the nature and essence of God, metaphysical questions as to how to reconcile the respective conception of God with views about the interaction of substances of fundamentally different kinds, and epistemological problems as to the possibility of knowledge and cognitive certainty than they were worried about whether the ultimate constituents of reality were mental or material elements.

However, if one were to situate their thoughts within the framework provided by Wolff it is not that difficult to find traces of idealism (both ontological and epistemological) in their respective positions. With respect to their metaphysical or ontological teachings, this claim may seem surprising. Whereas according to Wolff (ontological) idealists are representatives of a species of metaphysical monism Descartes is one of the most outspoken metaphysical dualists. Hence to impute idealistic tendencies to Descartes’ metaphysics looks like a mistake. And in the case of Spinoza one could argue that though he definitely is a (very radical) monist and thus could count as an idealist within Wolff’s taxonomy, he normally is considered to be rather a materialist in Wolff’s sense. Consequently, it appears as if already for conceptual reasons there is no basis to burden either Descartes or Spinoza with traces of metaphysical idealism a la Wolff. Leibniz, meanwhile, often seems unwilling to commit himself to ontological idealism even though that is the most natural interpretation of his monadology, while only Malebranche, as noted, seems to come close to explicitly asserting epistemological and perhaps ontological idealism as well.

Nevertheless, both Descartes and Spinoza provide a starting point for their metaphysical doctrines with their conceptions of God, a starting point that is already infected with idealistic elements if (ontological) idealism is understood as implying a commitment to the primacy or at least the unavoidability and irreducibility of mental items in the constitution and order of things in general. Both agree that in order to gain insight into the constitution of the world one has to find out what God wants us, or maybe better: allows us to know about it (see, e.g., Descartes: Meditations IV, 7–8 and especially 13; Spinoza: Ethics I, XVI). They also agree that the world is created by God although they have different views as to what this means. Whereas Descartes thinks of God as existing outside the world of the existing things He created (see Meditations III, 13 and 22) Spinoza holds that whatever exists is just a peculiar way in which God is present (see Ethics I, XXV, Corrolarium). Of all existing things all that God permits us to know clearly and distinctly is (again according to both Descartes and Spinoza) that their nature consists either in thinking or in extension. This claim can be seen as providing in the case of Descartes the basis for his justification of ontological dualism. His distinction between extended and thinking substances is not just meant to give rise to a complete classification of all existing things in virtue of their main attributes but also to highlight the irreducibility of mental (thinking) substances to physical or corporeal (extended) substances because of differences between their intrinsic natures (see, e.g., Meditations VI, 19, and Principles of Philosophy I, 51–54). In the case of Spinoza thinking and extension not only refer to attributes of individual things but primarily to attributes of God (s. Ethics II, Proposition I, II, and VII, Scholium), making them the fundamental ways in which God himself expresses his nature in each individual thing. This move gives rise to his ontological monism because he can claim that all individual things are just modes in which God’s presence is expressed according to these attributes.

Although the idea that God is the creator of the world of individual existing things (Descartes) or that God himself is manifested in every individual existing thing (Spinoza) might already be considered to be sufficient as a motivating force for subsequent disputes as to the true nature of reality and thus might have given rise to what were then called “idealistic” positions in ontology, other peculiarities within Descartes’ and Spinoza’s position might well have led to the same result, i.e., to ontological idealism. Especially their disagreement about God’s corporeality might have been such a motive. Whereas Descartes denies vigorously the corporeality of God (Principles of Philosophy I, 23) and hence could be seen as endorsing ontological idealism, Spinoza vehemently insists on God’s corporeality (Ethics I, Proposition XV, Scholium) and thus could be taken to be in favor of ontological materialism.

Things are different when it comes to epistemological idealism. It seems to be very difficult to connect Descartes’ and Spinoza’s views concerning knowledge with conceptions according to which knowledge has something to do with a cognizing subject actively contributing to the constitution of the object of knowledge. This is so because both Descartes and Spinoza think of cognition as a result of a process in which we become aware of what really is the case independently of us both with respect to the nature of objects and with respect to their conceptual and material relations. Descartes and Spinoza take cognition to be a process of grasping clear and distinct ideas of what is the true character of existing things rather than a process of contributing to the formation of their nature. According to Descartes the sources of our knowledge of things are our abilities to have intuitions of the simple nature of things and to draw conclusions from these intuitions via deduction (Rules for the Direction of the Mind III, 4 ff.). For him the cognitive procedure is a process of discovery (see Discourse on the Method, Part 6, 6) of what already is out there as the real nature of things created by God by finding out the clear and distinct ideas we can have of them (Discourse, Part 4, 3 and 7). In a similar vein Spinoza thinks of knowledge as an activity that in its highest form as intuitive (or third genus of) cognition leads to an adequate insight into the essence of things (Ethics II, Proposition XL, Scholium II, and Ethics V, Proposition XXV–XXVIII), an insight that gives rise to general concepts (notiones communes) on which ratiocinationes, i.e., the processes of inference and deduction, are based (Ethics II, Proposition XL, Scholium I) whose results provide the second genus of cognition (ratio). Thus the problem for both Descartes and Spinoza is not so much that of the epistemological idealist, i.e., to uncover what we contribute through our cognitive faculties to our conception of an object, rather their problem is to determine how it comes that we very often have a distorted view of what there is and are accordingly led to misguided beliefs and errors. Given what they take to be a basic fact that God has endowed us with the capacity to know the truth (albeit within certain limits), i.e., to know to a certain degree how or what things really are, this interest in the possibility of error makes perfectly good sense (Meditations IV, 3–17; Principles of Philosophy I, 70–72; Ethics II, Proposition 49, Scholium).

In his project for a “universal characteristic”, Leibniz can be regarded as having taken great interest in a method for inquiry, but he does not seem to have taken much interest in the epistemological issue of skepticism or the possibility of knowledge, and thus did not explicitly characterize his famous “monadology” as a form of epistemological idealism. But he did take a great interest in the ontology of substances, God the infinite substance and everything else as finite substances (in contrast to Spinoza, he rejected monism). Yet while the logic of his monadology clearly points toward ontological idealism, Leibniz frequently attempted to avoid this conclusion. One explicitly ontological argument for the monadology that Leibniz often deploys is that, on pain of infinite regress, everything composite must ultimately consist of simples, but that since space and time are infinitely divisible extended matter cannot be simple while thoughts, even with complex content, do not literally have parts, nor do the minds that have them, so minds, or monads, are the only candidates for the ultimate constituents of reality. Thus the late text entitled “The Monadology” begins with the assertions that “The monad which we are here to discuss is nothing but a simple substance which enters into compounds”, that “There must be simple substances, since there are compounds, [and] the compounded is but a collection or an aggregate of simples”, but that “where there are no parts, it is impossible to have either extension, or figure, or divisibility” and conversely where there is simplicity there cannot be extension or figure or divisibility (§§1–3). Yet monads must have some qualities in order to exist (§8) and to differ from one another, as they must (§9), and if the fundamental properties of matter are excluded, this leaves the fundamental properties of mind, which Leibniz holds to be perception, “The passing state which enfolds and represents a multitude in unity” (§14) and appetition, “the internal principle which brings about change or the passage from one perception to another” (§15; all from PPL, pp. 643–4). This argument clearly seems to imply that all finite substances are ultimately mental in nature (and the infinite substance, God, is obviously mental in nature), so it seems as if Leibniz ought to unabashedly affirm ontological idealism, from which epistemological idealism would automatically follow, since if there is knowledge of reality at all, which Leibniz hardly seems to doubt, and reality is ultimately mental, then knowledge too must be of the mental.

Yet Leibniz often seems to avert such a conclusion by appeal to his idea of “preestablished harmony”, and this is possible because he himself interprets this idea in two different ways. Early in his career, in such texts as “Primary Truths” (1680–84) and the “Discourse on Metaphysics” (1686) (both texts unpublished in Leibniz’s lifetime and not known to his immediate successors such as Wolff and Baumgarten), Leibniz introduces the doctrine of preestablished harmony on truth-theoretical grounds. His argument is that everything that is true of a substance is so because the predicate of a true proposition is contained in the complete concept of its subject and because that complete concept reflects the properties or “traces” in the substance that is that subject; that there are true propositions linking every substance in the world to every other, thus the complete concept of each substance must be a complete concept of the universe itself and each substance must bear within itself as properties traces of every other in the universe; and thus that each substance must reflect, or, as mental, represent the entire universe. Yet since (finite) substances are also defined as existing independently of one another (although not existing independently from the infinite substance, God), there is a question as to why each should truthfully represent all the others, which Leibniz answers by appeal to the idea of a preestablished harmony: although considered from the point of view of the concept of substance it does not seem necessary that every substance truly represent all the others, in his goodness, thus in his preference for a maximally harmonious world, God has nevertheless made it such that they do.

In this mood, Leibniz tends to explain the existence of body as an artifact of the fact that each monad represents the world from its own point of view: physical locations and the bodies that occupy them are just the way in which the difference in the points of view of the monads is represented by them, but have no deeper reality; or, as Leibniz often says, space, spatiality, and bodies are just phenomena bene fundata, i.e., “well-founded modes of our consideration” (PPL, p. 270).

However, sometimes Leibniz writes as if space and time are not merely the way in which the pre-established harmony among monads presents itself to (their) consciousness, but as if the mental and physical or extended are two separate realms, each evolving entirely in accordance with its own laws, but with a pre-established harmony between them creating the appearance of interaction. Perhaps Leibniz was genuinely undecided between two interpretations of the pre-established harmony and two conceptions of the reality of body, sometimes being a committed idealism and sometimes a dualist. (As we will see later, even among the most committed absolute idealists of the nineteenth century it is not always clear whether they are actually denying the existence of matter or only subordinating it to mind in one way or another).

Leibniz’s monadology could thus be seen as a forerunner of both epistemological and ontological idealism, and his conception of space and time as phenomena bene fundata was clearly a forerunner of Kant’s transcendental idealism. But as we have just seen, he did not himself unequivocally affirm idealism, and as we will shortly see subsequent Leibnizians such as Alexander Baumgarten argued for dualism and for a corresponding interpretation of pre-established harmony. Nicolas Malebranche was also a dualist, committed to the existence of both mind and body, and an occasionalist, who held that since causation is necessary connection and the only truly necessary connection is between God’s intentions and their effects, bodies cannot directly cause modifications of minds (or each other) but rather there can be a causal relation between body and mind only if God intends the mind to undergo a certain modification upon the occasion of a certain change in a body (hence the term “occasionalism”). His further doctrine that the mind sees all things in God, however, depends on his particular view of what modifications the mind undergoes in perception. He holds that sensations are literally modifications in the mind, but that they are highly indeterminate, or in later terminology lack determinate intentional objects, and that genuine understanding occurs only when and to the extent that the determinate ideas in the perfect intellect of God are disclosed to finite, human minds, to the extent that they are. Malebranche’s position can be considered a theological form of Platonism: Plato held that the true Ideas or Forms of things have a kind of perfection that neither ordinary objects nor representations of them in human minds do, and therefore must exist someplace else; Malebranche takes the obvious further step of supposing that perfect ideas can exist only in the perfect intellect of God. He then supposes that human thought is intelligible to the extent that these ideas are disclosed to it, on the occasion of various sensations themselves occasioned by God but not literally through those sensations. The crucial point is that genuine understanding consists in the apprehension of ideas, even though these are literally in the mind of God rather than of individual human beings, rather than of physical objects, even though the latter do exist. Malebranche had significant influence on both Berkeley and Hume, though neither the former and certainly not the latter accepted his position in its entirety. His position that knowledge consists in individual minds apprehending ideas in some greater mind would also be recreated by idealists as late as T.H. Green and Josiah Royce in the second half of the nineteenth century, as we will later see.

Before we turn to British or Anglophone versions of idealism, earlier or later, one last word about idealism within pre-Kantian rationalist philosophy is in order. As earlier mentioned, dualism rather than ontological idealism became the default position of the German successors to Leibniz, the so-called “Leibnizo-Wolffians” who dominated the teaching of philosophy in many German universities from the second or third decade of the eighteenth century until the time of Kant and in some cases even beyond, and they correspondingly opted for the interpretation of the pre-established harmony as a relation between minds and bodies rather than among minds or monads alone. It may also be noted that defending dualism by means of an explicit “refutation of idealism” became the norm among these philosophers. This may be seen in Alexander Gottlieb Baumgarten’s Metaphysica of 1739, which would become Kant’s textbook for his lecture courses in metaphysics and “anthropology” (empirical psychology) until the very end of the eighteenth century. Baumgarten accepts that the ultimate constituents of the world must be simples, hence monads of some kind. But he does not suppose that monads are necessarily minds or intellects, hence a dualism of monads is at least possible. Idealism would be the position that there are only intellectual monads; he says that

An intellectual substance, i.e., a substance endowed with intellect, is a spirit (an intelligence, a person)…. Whoever admits only spirits in this world is an idealist. (Metaphysics, §402, pp. 175–6)

Baumgarten follows Wolff in distinguishing between two possible forms of idealism, first egoism, which admits the existence of only one spirit, that of the person contemplating such a doctrine, and then idealism proper, which allows the existence of multiple spirits. But both are refuted by the same argument. This argument builds on a Leibnizian principle not hitherto mentioned, the principle of plenitude, or the principle that the perfection of the most perfect world, which is the one that God created, consists in the maximal variety of the universe compatible with its unity or coherence (e.g., “Monadology”, §58, PPL, p. 648), which was in turn the basis of one of Leibniz’s arguments for the identity of indiscernibles. Baumgarten then argues simply that a universe that contains not only more substances but also more kinds of substances rather than fewer is a more perfect universe, and necessarily exists in preference to the other; and a universe that contains not only multiple minds rather than a single mind but also bodies in addition to minds is therefore a more perfect universe than either of the former would be, and is the kind that actually exists. In his words,

the egotistical world, such as an egoist posits, is not the most perfect. And even if there is only one non-intellectual monad possible in itself that is compossible with spirits in the world, whose perfection either subtracts nothing from the perfection of the spirits, or does not subtract from the perfection of the spirits so much as it adds to the perfection of the whole, then the idealistic world, such as is posited by the idealist, is not the most perfect, (Metaphysics, §438, p. 183)

and hence not the kind of world that exists. No one outside of the immediate sphere of Leibnizianism would ever again proffer such a refutation of idealism. But both Baumgarten’s recognition of idealism and his refutation of it in a university textbook make it clear that by the middle of the eighteenth century idealism had become a standard topic for philosophical discussion, a position it would retain for another century and a half or more.

3. Idealism in Early Modern British philosophy

The relation between ontological and epistemological idealism is complex. Ontological idealism can be argued for on its own, and bring epistemological idealism in its train. Epistemological idealism can be argued for independently of ontological assumptions but lead to ontological idealism, especially in the hope of avoiding skepticism. Or epistemological idealism can be the basis for rejecting any pretenses to ontology, including ontological idealism. The first option may have been characteristic of some rationalists, such as Leibniz in his more strictly idealist mood. Both of the latter two are found within early modern British philosophy. We find considerations pushing toward epistemological idealism in both Hobbes and Locke in spite of the avowed materialism of the first and dualism of the second, who therefore obviously did not call themselves idealists. Berkeley argues for epistemological idealism and then adds ontological idealism in order to avert skepticism, although he calls his position immaterialism rather than idealism. Hume, by contrast, although calling himself neither an immaterialist nor an idealist, nevertheless adopts arguments for epistemological idealism similar to some of Berkeley’s, but then uses that position as the basis for a critique of traditional metaphysical pretensions, including those to ontological idealism—while also being drawn to ontological idealism in resistance to what he regards as the natural tendency to dualism. Hume’s critical attitude toward metaphysics is subsequently taken up by Kant, although Kant famously asserts on practical grounds some of the very same metaphysical theses that he argues cannot be asserted on theoretical grounds.

The British philosophers were all hostile toward dogmatic metaphysics in Wolff’s sense, although until the time of Hume, who had some familiarity with Leibniz, the metaphysics with which they were familiar were those of Descartes, Aristotelian scholasticism, and Neo-Platonism, which had become domesticated in Britain through the work of the Cambridge Platonists in the second half of the seventeenth century. All of these movements fed into the general movement of rationalism, while the British philosophers, typically lumped together under the rubric of empiricism in spite of their own differences, all believed, albeit for different reasons, that the doctrines put forward by dogmatic metaphysicians rest on a totally unfounded conception of knowledge and cannot survive rational scrutiny (empiricists might themselves be considered critical rationalists). Thus the primary task of philosophy for these philosophers became that of providing a theory of knowledge based on an adequate assessment of the constitution of human nature, for they were interested in knowledge only as a human achievement. However, it is not human nature in general that is of interest in this context but the workings of those human powers or faculties that are responsible for our human ability to relate to the world in terms of knowledge-claims. (Thus Kant’s attempt to argue on practical grounds for metaphysical theses that could not be justified on theoretical grounds would be a major departure from the methods of the British empiricists.) These faculties were attributed by the British as well as their Continental opponents to what was called “the mind” (mens, consciousness, Bewußtsein), an attribution which resulted in moving the “operations of the mind” into the center of philosophical attention. Reflections on the conditions of the possibility of knowledge led Hobbes and Locke to what might be considered forms of epistemological idealism in spite of their ontological commitments to materialism or dualism respectively, while Berkeley concluded that their epistemological idealism would lead to a skepticism that could be avoided only by his own more radical ontological idealism. Hume’s position remains complex and for this reason controversial. His thesis that our beliefs in causation, external objects, and even the self are all founded on “custom” and imagination rather than “reason” may be considered a form of epistemological idealism, but while he sometimes seems to attempt to avoid commitment on ontological questions altogether, at other times, as in his argument that the existence of external objects in addition to our impressions is only a fiction, he seems to infer ontological idealism from his epistemological idealism. In spite of their differences, almost all British philosophers from Hobbes up to and including Hume insisted that the highest priority for philosophy is to give an analysis of the conditions and the origin of knowledge, while they gave not only somewhat different accounts of what these conditions consist in and how they contribute to a convincing story about the origin of knowledge but they also had to face quite interesting “metaphysical” consequences from their respective accounts.

This is easily confirmed by looking briefly at some of their main convictions concerning knowledge, starting with Thomas Hobbes (1588–1679). As Hobbes points out in the chapters Of Philosophy and Of Method in the first part (Computation or Logic) of the first section (Concerning Body) of his Elements of Philosophy (1655), knowledge is the result of the manipulation of sensory input based on the employment of logical rules of reasoning (ratiocination) in acts of what he calls “computation”. He describes the details of this process most succinctly in a short passage in chapter 6 of the first part (Human Nature) of his The Elements of Law, Natural and Politic (1640), his first major philosophical work. After distinguishing what he calls “sense, or knowledge original” from “knowledge … which we call science”, he goes on to “define” knowledge “to be evidence of truth, from some beginning or principle of sense” and formulates four principles that are constitutive of knowledge:

The first principle of knowledge therefore is that we have such and such conceptions; the second, that we have thus and thus named the things whereof they are conceptions; the third is, that we have joined those names in such manner, as to make true propositions; the fourth and last is, that we have joined those propositions in such manner as they be concluding.

The message is straightforward with respect to both the basis and the formation of knowledge: senses (sensations) are basic to our acquisition of knowledge in that they lead to conceptions (representations) to which we attach names (concepts) which we then put together into propositions which, if true, already constitute knowledge, and from which there arise further knowledge if we draw conclusions in an orderly way from them.

Though the account given by Hobbes of the origin and the formation of knowledge is rightly called empiricist because it traces all knowledge back to the senses or sensations and their non-sensory causes, i.e., to what he calls “things without us”, it is by no means directly committed to any sort of ontological idealism or dualism; on the contrary, Hobbes’s preferred ontological position is materialism. Nevertheless, his account may lead to an early form of epistemological idealism. This is so because although Hobbes makes no claims as to either the constitution and the reality of what causes sensations or to any specific contribution on the part of the subject of knowledge to what we take to be the “accidents or qualities” of objects, he states, again most explicitly in the part on Human Nature in The Elements of Law, (1) that there are causes of our sensations which by way of their motions give rise to what we sense as qualities, but (2) that these qualities only have the status of “seemings and apparitions”. In his own words: “The things that really are in the world without us, are those motions by which these seemings are caused” (Elements of Law, Part I, chap. II, 10). While he is confident that there are external objects, and thus has no intention of affirming ontological idealism, nevertheless because in Hobbes’s opinion we could have conceptions of these seemings even if there were no objects around (ibid. chap. 1, 8) there is for him no basis on which to found any metaphysical claims to the real existence of an external world or any epistemological basis for claiming knowledge of the real constitution of a subject-independent world or its real existence. Thus, Hobbes’s position is best described as agnosticism when it comes to metaphysics forced on him by a form of epistemological idealism. This is nicely confirmed by a passage from part II (The First Grounds of Philosophy) where he declares:

Now things may be considered, that is, be brought into account, either as internal accidents of our mind, in which manner we consider them when the question is about some faculty of the mind; or as species of external things, not as really existing, but appearing only to exist, or to have a being without us [emphasis added]. And in this manner we are now to consider them. (chap. VII, 1)

In spite of a pre-reflective disposition toward dualism, an explicit argument for an agnostic attitude with respect to the ultimate constitution of reality, thus a form of epistemological idealism without any argument for ontological idealism, is also characteristic of John Locke (1632–1704). Already in The Epistle to the Reader of An Essay concerning Human Understanding (1690) he denounces rationalist metaphysics as a “Sanctuary of Vanity and Ignorance” and declares in the first book of his Essay right at the outset:

I shall not at present meddle with the physical consideration of the mind; or trouble myself to examine, wherein its essence consists, or by what motions of our spirits, or alterations of our bodies, we come to have any sensation by our organs, or any ideas in our understandings; and whether those ideas do in their formation, any, or all of them, depend on matter or no [emphasis added]: These are speculations, which, however curious and entertaining, I shall decline. (Book I, chap. I, 2; s. also Book II, chap. XXI, 73)

Instead he restricts his investigation to the “purpose to enquire into the original, certainty, and extent of human knowledge” (ibid.). Such an investigation presupposes an acquaintance with our own minds, and thus according to Locke the most pressing task is to understand the mind or the understanding itself. And because for Locke the sole material the mind has the ability to process are ideas, the most pressing task if one wants to understand the possibility of knowledge is to give an account of “how he [the mind] comes by them [the ideas]” (Book II, chap. I, 1). There is no need to go into the details of Locke’s conception of how the mind gets ideas and what the understanding does with them in order to arrive at knowledge. Though his description of these processes differs in some interesting ways from the model Hobbes proposes, in the end both Hobbes and Locke share the view (1) that whatever we can know depends on our having ideas which must be somehow based in sensation, (2) that there must be some external cause (Hobbes) or some source of affection (Locke) which gives rise to sensory ideas, yet (3) ultimately we are ignorant about the real constitution of these causes and these sources. What we know is the content and structure of our own ideas (epistemological idealism), although we have no reason to deny the existence of external objects (thus to assert ontological idealism) and even assume that in some regards external objects resemble our ideas of them (in the case of primary qualities).

Obviously it is mainly point (3) that is of importance for the question of how much dualism or ontological idealism is involved in Locke’s version of the operations of the mind. Again, as in the case of Hobbes, it seems that Locke’s position is meant to be neutral against and compatible with all these alternatives and that he wishes to stay agnostic with respect to them. This is indicated especially well by his theory of substance and his remarks concerning the limits of knowledge. Substances, Locke famously holds, “are such combinations of simple Ideas, as are taken to represent distinct particular things subsisting by themselves” (II, XII, 6). If one analyzes our concept of a substance one

will find he has no other Idea of it at all, but only a Supposition of he knows not what support of such qualities, which are capable of producing simple Ideas in us. (II, XXIII, 2)

The reasons for this supposition are two: (1) we cannot make sense of the concept of an unsupported quality or of ideas subsisting by themselves, (2) we know from experience that “a certain number of these simple Ideas go constantly together” or “exist together” (II, XXIII, 2 and 3). Though Locke thinks of these reasons as totally compelling, he sees quite well that they do not justify any claim as to what a substance or a thing really is, what its nature or constitution consists in. Thus he never gets tired of emphasizing that we only have a confused idea of substance (a claim also made by Leibniz about three-quarters of our knowledge, although he held that we have a clear concept of what substance is), and repeats quite often (at least three times in Book II, chap. XXIII alone) that

Whatever therefore be the secret, abstract nature of substance in general, all the ideas we have of particular distinct sorts of substances, are nothing but several combinations of simple ideas, co-existing in such, though unknown, cause of their union, as makes the whole subsist of itself. (II, XXIII, 6)

He restricts this agnostic attitude not just to corporeal substances or bodies but extends it to spiritual substances or minds as well:

It is plain then, that the idea of corporeal substance in matter is as remote from our conceptions and apprehensions, as that of spiritual substance or spirit; and therefore from our not having any notion of the substance of spirit, we can no more conclude its non-existence, than we can for the same reason deny the existence of body; it being as rational to affirm there is no body, because we have no clear and distinct idea of the substance of matter, as to say there is no spirit, because we have no clear and distinct idea of the substance of a spirit. (II, XXIII, 5)

This criticism of any metaphysical claims concerning the ultimate constitution of reality is accompanied by a more general warning against the overstepping of the natural limits of our cognitive faculties. According to Locke it is just a fact about human nature that there are limits to the powers of the understanding. These powers are meant to be bestowed to us by God to an extent sufficient for us to know “Whatever is necessary for the Conveniences of Life, and Information of Virtue” (I, I, 5; s. also II, XXIII, 12) but only to that extent. If therefore the nature and the constitution of substances both corporeal and spiritual are beyond our cognitive grasp then we should take this to be a hint that God has set limits to what we can know because he sees no reason for us to know everything. Even if the powers He endowed on us would be magnified infinitely we still would remain clueless as to what substances really are because we still would be stuck in a world of qualities (this is one way of reading II, XXIII, 12). Thus, in the end metaphysical knowledge of any kind is meant to be beyond our reach. This, however, is nothing we should be concerned about:

For, though the comprehension of our understandings comes exceeding short of the vast extent of things; yet we shall have cause enough to magnify the bountiful author of our being, for that proportion and degree of knowledge he has bestowed on us, so far above all the rest of the inhabitants of this our mansion. (I, I, 5)

For Locke, epistemological idealism combined with ontological agnosticism is an expression of piety. Locke’s position may be regarded as a theological expression of the most fundamental motivation for epistemological idealism: no matter how much we know about objects and at what level of detail, we still know them only from our own, human point of view.

The agnosticism with respect to the ultimate constitution of substances and things or of the fundamentum in re of “the ideas thereof” characteristic of Hobbes and Locke is challenged forcefully by George Berkeley (1685–1753), for whom their agnosticism becomes a form of skepticism and even impiety. In his Treatise Concerning the Principles of Human Knowledge (1710) he raises doubts about whether an agnostic stance along the lines of Hobbes and Locke can be upheld consistently if one thinks about the origin and the properties of ideas the way they do. Although in his Treatise Berkeley does not mention Hobbes at all and addresses Locke not by his name but by formulas like “esteemed philosopher” and “learned author” (Treatise, Introduction, 11) very few times, it is abundantly clear that he wants to confront especially Locke with an ugly alternative: either his conception of a substance or a thing has “no distinct meaning annexed to” it (Treatise, I, 17) and is nonsense, or he has to endorse not just epistemological idealism but ontological idealism as well. In other words, Berkeley’s point is that Locke cannot afford to be agnostic with respect to the metaphysical status of substances and things if he wants us to think of ideas as the immediate objects of human knowledge.

Berkeley’s position is thus that ontological idealism must be accepted along with epistemological idealism. His arguments in favor of ontological idealism based on the acceptance of ideas as the objects of human knowledge are rather straightforward, turning on presuppositions which he at least considered uncontroversial. Although his taxonomy of the different kinds of ideas deviate in ways that are not of interest here from Locke’s classification, he agrees with Locke that ideas exist only “in the mind” (Treatise, I, 2). He takes the mind to be a “perceiving, active being” which itself is not “any one of my ideas, but a thing entirely distinct from them, wherein they exist or, which is the same thing, whereby they are perceived” (ibid.). From these stipulations he derives his most fundamental and famous claim (1) that “the existence of an idea consists in being perceived” (ibid.) or that “their esse is percipi” (Treatise, I, 3) by the perceiving, active mind. Already here Berkeley has the means in place to cast into doubt the meaningfulness of the assumption that there might exist unperceived objects or things. This is due to his restriction of existence to what is perceivable or, even narrower, to what is perceived: If the only objects that exist for a mind—whether it is my own mind or the mind of other human beings or the divine mind—are ideas because there is nothing else that can exist for the mind, then the very concept of something that exists but is not for the mind or is unperceived is a contradiction in terms. Thus if, as Berkeley supposes Locke does, one thinks of things as consisting of collections of ideas, he asks how could one take a thing to be something other than ideas and nevertheless to exist? This question underlies Berkeley’s confidence in what is often referred to as his “master argument”, the argument that one cannot conceive of anything existing unconceived because in trying to do so one is still conceiving of the object (Treatise, I, §23). This seems open to the obvious objection that he is confusing the content of a proposition (for example, “The earth may still exist after the extinction of all conscious life”) with the act of entertaining (“conceiving”) such a proposition, which of course cannot take place except in a conscious being; but if he is already committed to the thought that objects of knowledge are nothing but ideas, it is at least understandable that he should overlook this distinction.

The second conviction, also meant to be damaging to Locke’s view about substances, on which Berkeley rests his case in favor of idealism is the claim (2) that “an idea can be like nothing but an idea” (Treatise, I, 8). Although this claim is initially put forward in the context of his well-known criticism of Locke’s primary-secondary-quality distinction, it is equally relevant for his denial that there are things “without the mind”. The reasoning on which this claim is based seems to be the following: For two items to stand in the relation of likeness they must have something in common. However, if an idea is mind-dependent and if ideas are all there is for the mind, then what is “without the mind” must be different in every respect from an idea. Thus a relation of likeness cannot obtain between ideas “in the mind” and things “without the mind”. Berkeley puts this point quite bluntly by appealing to observation:

If we look but ever so little into our thoughts, we shall find it impossible for us to conceive a likeness except only between our ideas. Again, I ask whether those supposed originals or external things, of which our ideas are the pictures or representations, be themselves perceivable or no? if they are, then they are ideas, and we have gained our point; but if you say they are not, I appeal to any one whether it be sense, to assert a colour is like something which is invisible; hard or soft, like something which is intangible; and so of the rest. (Treatise, I, 8)

There is a third claim that is essential to both Berkeley’s criticism of Locke and the idealistic position he is going to adopt. This is the claim (3) that ideas are passive and causally inert, i.e., they can neither produce nor alter another idea (Treatise, I, 25). This claim he also bases on observation:

whoever shall attend to his ideas, whether of sense or reflection, will not perceive in them any power or activity; there is therefore no such thing contained in them. A little attention will discover to us that the very being of an idea implies passiveness and inertness in it, insomuch that it is impossible for an idea to do any thing, or, strictly speaking, to be the cause of any thing. (ibid.)

Again the primary function of this claim is to discredit a Lockean view according to which we have to think of the primary qualities of things—which are contents of the most fundamental ideas we have of them—as the causes of sensations or of sensory ideas. It is, however, also meant to support the untenability of the assumption that agnosticism with respect to the real existence of mind-independent things is a viable option for a believer in Locke’s model of how and by what means we acquire knowledge of objects.

Berkeley’s criticism of Locke’s theory concerning substances is not carried out for its own sake. On the contrary, it is meant to establish what Berkeley thinks to be the unavoidable metaphysical consequences of a position that takes ideas “in the mind” to be the only material for the operations of the mind in its acquisition of knowledge. These metaphysical consequences consist in a thoroughgoing idealism (a term, however, never used by Berkeley himself, who preferred to call his position “immaterialism”) with respect to the nature and constitution of things or substances. Berkeley’s way of establishing this result is open to many questions. However, the basic outline of his overall argument can be sketched thus: If existence is restricted to ideas (and minds) and if, what is undoubtedly the case, things or substances exist, then things or substances must be ideas (or minds) too. Now, as Locke has convincingly shown, we can have ideas of particular things or substances, e.g., gold and lead, humans and sheep, distinguished by our ideas of their various properties, but we have only a confused or obscure idea of substance in general, which we suppose to underlie whatever collection of ideas we take to be a thing or a substance of one kind or another. But if we cannot have any ideas of things or substances other than our ideas of their properties, which clearly exist in minds, then the only clear ideas of things that we have is as ideas, and in that case, if they do not seem to exist in our own individual, human minds, then things or substances must be ideas in some other non-human, i.e., divine mind. This divine mind cannot be itself an idea because it must be conceived as an active principle that can be the cause of ideas, a principle of which we can have no idea but only a “notion” (Treatise, I, 26, 27). Therefore, the very fact that we take things or substances to be real commits us to the claim that things are ideal entities perceived by the mind of God. Ontological idealism, one could say, is the only tenable basis for a realistic stance for Berkeley, but it leads to a realism about minds, human and divine, rather than of what he always calls material substance. And if one is to accept his re-interpretation of causality as a purported relation between ideas in terms of his theory of marks and signs, in particular his theory that what we think of as ideas of objects are signs of (God’s plan for) future possible ideas for us (cf. Treatise, I, 65 f.), then one also has to agree to epistemological idealism. But again, for Berkeley epistemological idealism without ontological idealism, the theory that all that exists are minds and their ideas, would be a form of skepticism.

Up until the point at which he introduces the mind of God into his argument, all of Berkeley’s considerations in behalf of epistemological idealism might be thought of as expressions of the basic insight that we can only conceive of reality from our own point of view, which are then extended into ontological idealism in order to avoid the whiff of agnosticism or skepticism and supplemented with the existence of a divine mind in order to satisfy an ineliminable tendency to believe in the existence of something more than one’s own mind or even of human minds in general. We will later see that the tendency to preserve both the impulse to idealism and the conviction that there is something more than ordinary human minds by positing a more than human mind is characteristic of many versions of idealism until the end of its glory days at the beginning of the twentieth century. This tendency is decidedly absent from the philosophy of David Hume, however.

David Hume (1711–1776) learned a great deal from Berkeley, above all his empiricist epistemology, but for the most part he tried to avoid Berkeley’s outright commitment to ontological idealism. Hume’s view that our knowledge consists of our ideas, our recognition of “philosophical” relations among them, such as identity and difference, and our recognition of “natural” relations among them such as causation, which are established by imagination and custom, can be considered a form of epistemological idealism—causality, in particular, which Hume regards as the basis of all our knowledge of existence, is at the same time reduced to a way of feeling and thinking, in other words a state of mind. But depending on how he is read, Hume either accepts the skepticism about possible external objects that Berkeley tries to avoid with an ontological idealism that renders any external objects other than other human or divine minds impossible, or else holds that even if there are valid arguments for skepticism it is psychologically impossible for human beings to remain in a skeptical frame of mind, thus we naturally even if not rationally believe in the existence of objects apart from our ideas of them. However, in those passages, prominently in Book I, Part IV of his early Treatise of Human Nature (1739–40), where Hume entertains a kind of monism that sees both “minds” and “objects” as nothing but different sets or “bundles” of one sort of thing, namely, perceptions, impressions and their paler copies, ideas, his position might seem much like Berkeley’s ontological idealism, with the difference that while he reduces all reality to mental states like impressions and ideas he does not see these as properties that must inhere in substantial minds any more than in substantial bodies, both of which are fictions we introduce in order to explain continuities among those impressions or ideas (although it may be difficult to explain who is introducing those fictions without resorting to substantial minds after all).

Hume’s version of epistemological idealism with regard to causation is clearly on view in his 1748 Enquiry concerning Human Knowledge, which was quickly translated into German and would eventually provide Kant with the stimulus for his own aprioristic rather than empiricistic version of epistemological idealism with regard not only to causation but to all of what he called the categories of pure reason, including especially substance and interaction as well as causation. But since Kant was not familiar with the contents of Hume’s earlier Treatise of Human Nature, he did not know that Hume too had generalized his approach to causality to the cases of mind and body, nor did he know that Hume may have tried to sidestep Berkeley’s commitment to substances but not his ontological idealism altogether by his theory of both minds and bodies as bundles of perceptions. Kant would try to avert Berkeley’s ontological idealism by a different stratagem, but before we come to that we must consider Hume’s position more fully. Hume accepted from Locke and Descartes before him that the immediate objects of consciousness are what they had called ideas, although he reserves that word for copies or subsequently recalled perceptions rather than the originally experienced perceptions that he calls impressions. He also adopts the view of his predecessors that knowledge lies in the recognition of relations among impressions, ideas, or both, and divides those relations into two kinds, philosophical and natural. Philosophical relations are those immediately evident on reflection on or comparison of particular ideas, and include resemblance, identity, spatial and temporal relations such as above and below or before and after, number and degree, and logical contrariety (Treatise of Human Nature, I.I.5), while natural relations are those that are not immediately evident on reflection on a single impression or idea or in a single comparison of any number, but which instead become evident, or more properly are formed, only through repeated experience. Hume’s best known argument is then that causation is not a philosophical but a natural relation: the causal relation is comprised by temporal succession, spatial contiguity, and necessary connection, and while the first two are philosophical relations that are immediately apparent, the necessary connection between different ideas—those of a cause and its separate effect—is, unlike the necessary identity of two qualitatively similar ideas, not immediately apparent, as Hume puts it, to reason (I.III.2), but instead grows only out of repeated experience, the repeated experience of qualitatively similar pairs of impressions which causes them to become linked in the mind, as we would ordinarily say, or at least in consciousness, as the careful Hume should say at most (I.III.6). In fact, Hume’s argument is that repeated experience itself has two effects: it creates a habit of thought such that upon the presentation of an impression of one kind that has repeatedly been experienced in spatial and temporal conjunction with one of another kind, a vivid version of the idea of the kind of impression with which the first kind of impression has been repeatedly associated immediately occurs—this is the essence of causal inference or belief, because a belief is nothing but an idea that is almost as vivid and forceful as the impression of which it was once a copy (I.III.7–8)—and further, there is an actual feeling of the mind (as we would ordinarily say) being tugged from the one impression to the other idea—this is the basis of the idea of necessary connection, a connection which the mind then “spreads” upon its objects to form the idea of a necessary connection among them or their states (I.III.14).

Hume’s theory is a form of epistemological idealism in that it relocates the relation of causation from the external objects where we would ordinarily suppose it to obtain to the mind, which we would ordinarily suppose knows but does not constitute the relation known. In Hume’s words,

Tho the several resembling instances, which give rise to the idea of power, have no influence on each other, and can never produce any new quality in the object, which can be a model of that idea [of power or causation], yet the observation of this resemblance produces a new impression in the mind, which is its real model…. Necessity, then, is the effect of this observation, and is nothing but an internal impression of the mind, or a determination to carry our thoughts from one object to another. (I.III.14, para. 20)

Several things may be noted about this theory. For one, if it had been Hume’s intent to raise a general skepticism about causation, based on the famous worry about induction that he himself raises, especially in the subsequent Enquiry, namely that an assertion of causality claims that future impressions will occur in the same patterns as past ones but there is no basis “in reason” for assuming that the future will resemble the past, then the relocation of causation from the domain of objects to the domain of the mind should make no difference, because we have no more reason to believe that the mind will behave the same way in the future as it has in the past than we do to believe that about anything else. So we must either believe that Hume is very confused, not realizing that his skepticism about induction as applied to external objects must undermine our confidence in his application of induction to the mind itself, or else that he is very arch, and that he means us to do his skeptical work for him by carrying over skepticism about induction to the case of the mind itself, or else that he is not really worrying about issues of justification and thus of the threat of skepticism at all, but just means to be giving a plausible description of the only possible basis for causal inference, namely the mind’s experience of itself. The last possibility may well seem to be the most plausible, leading to the “naturalist” reading of Hume promoted by Norman Kemp Smith and Barry Stroud rather than the “skeptical” reading of Hume accepted by his contemporaries such as James Beattie and defended recently by Robert Fogelin.

There is a further issue with Hume’s treatment of causation that is largely suppressed in the Enquiry but that was evident in the Treatise, namely, that although, as we saw in the last passage quoted, Hume sometimes presents his epistemological idealism about causation by describing necessary connection as being displaced from the object to the mind, on his own strict interpretation of empiricism there is a problem in positing the existence of either objects or minds distinct from perceptions. This is what pushes Hume towards his own form of ontological idealism. That is, although we naturally speak of perceptions as being of objects and in or by the mind, on the view that all knowledge is founded on perception and that in perception we are immediately acquainted with nothing but perceptions, it becomes problematic how we could have knowledge either of the mind itself or of any object of perceptions distinct from those perceptions. Hume puts the former point succinctly by arguing that we have no perception of the self distinct from our perception of its perceptual states:

For my part, when I enter most intimately into what I call myself, I always stumble on some particular perception or other, of heat or cold, light or shade, love or hatred, pain or pleasure. I can never catch myself at any time without a perception, and never can observe any thing but the perception. (Treatise I.IV.6.3)

He then argues that in fact the self is

nothing but a bundle of different perceptions, which succeed each other with an inconceivable rapidity, and are in a perpetual flux and movement. (I.IV.6.4)

and that the idea of a continuous self is but a fiction or illusion created by relations of resemblance and continuity among perceptions in the bundle, just as both the idea of and belief in causal connection were created by repetition of pairs of impressions. Without saying that the objects of perception are also nothing but bundles of related perceptions, Hume presents a similar account of how the idea of objects distinct from our perceptions of them is generated by our impression of continuity among perceptions: although only philosophers reflect on this, in fact we know that perceptions are fleeting and transitory; we mistake continuity among them for enduring identity; and we then invent something other than perceptions, something not fleeting and transitory, to which to ascribe that enduring identity (Treatise, I.IV.2). In neither case, however, do we actually have a clear idea of any object or substance distinct from our perceptions: we do not have such an idea of external objects or their substance, but neither do we have a clear idea of the mind or its substance. The only ideas we have are copies of our impressions, or perceptions.

Hume’s attack on the supposition that we have an idea of the mind as distinct from its impressions thus constitutes a rejection of Berkeley’s commitment to the existence of mental substances, but not of ontological idealism altogether. On Hume’s account, we are not entitled to assert the existence of both ideas and the minds, human or divine, that have them, but only the former. At the same time, he does not seem to think that we are forced into skepticism about either minds or external objects by his approach, that is, into a position that there may really be minds and external objects but we cannot know that fact or their real qualities; yet he still has a lingering worry that although there are psychological mechanisms leading us to form the fictions of minds and bodies beyond perceptions, we do not really know what we are talking about when we talk about such things, and thus cannot even coherently doubt whether we have knowledge of them—our talk about them is explicable but meaningless. Hume thus seems to end up with an uneasy compromise between epistemological idealism and ontological agnosticism, on the one hand, and his own form of ontological idealism on the other.

4. Kant

The first major philosopher actually to call himself an idealist was Immanuel Kant (1724–1804), although as soon as he did so he labored to distinguish his position from Berkeley’s, not using these terms but essentially identifying himself as an epistemological but not ontological idealist. The sources as well as the form of Kant’s idealism are complex. Kant was deeply impressed by what he knew of Leibniz (many of the texts that are crucial to later understandings of Leibniz, such as “Primary Truths”, having been unknown in Kant’s times, or others, such as the New Essays on Human Understanding, having been published only when he was well into his career) and the view that space and time are phaenomena bene fundata and by what he knew of Hume and the view that causation is a form of thinking that we impose upon our experience rather than something we directly experience it. He was more generally impressed by the empiricist argument that our knowledge of objects depends upon experience of them. However, he thought that both the Leibnizian and the Humean approaches failed to account for the possibility of synthetic a priori knowledge, that is, knowledge that goes beyond the mere analysis of concepts, thus does more than merely unpack explicit or tacit definitions, but yet legitimately claims universal and necessary validity. But, unlike Plato, the original apriorist avant la letter, he does not see synthetic a priori knowledge as leading to realism about objects having the features that we know a priori, nor, like Malebranche, the theological Platonist, does he see such knowledge as knowledge of the mind of God; rather, he sees it as providing the conclusive argument for epistemological idealism through the premise that we can only know to be necessary and therefore universally valid the forms that we ourselves impose upon our experience. At the same time, even though when he wrote his main works he was not well-informed on the aporia about subjects and objects about which Hume had ultimately thrown up his arms in the Treatise, which has here been characterized as the tension in Hume between agnosticism and ontological idealism, Kant recognized that we cannot talk about what he called appearances without conceding the real existence of subjects to which objects appear as well as the objects that appear to such subjects. Kant was thus led to what he called “transcendental idealism”, a position that combines epistemological idealism about the main structures of objects, that is, the view that we ourselves impose spatiality, temporality, substantiality, causality, and other forms upon our experience and precisely because we know these forms a priori cannot regard them as also the real forms of objects independent of ourselves, with a kind of ontological realism, the view that in some sense our selves and our objects really do exist independently of our representations of them. Though he identifies his own “transcendental idealism” with “empirical realism” he does not want to call his own position “transcendental realism”, because for him that would be the view that objects independent of our representations do exist with the forms that we represent them as having, thus a form of epistemological realism rather than idealism. Neither would he even be happy to call that part of his position ontological idealism in the sense in which we have been using that terminology here, because it is part of his position that, at least from a theoretical point of view, we cannot suppose that even our own minds are really as they appear to us, nor can we assert that the reality that ultimately underlies the appearance of minds is essentially different from the reality that ultimately underlies the appearance of bodies. But he is confident that we are entitled to assert the existence of some sort of reality underlying the appearance of both minds and bodies, so that epistemological idealism must be accompanied by some sort of ontology, even if only an indeterminate one.

Kant had already published a number of substantial scientific as well as philosophical works before the “great light” of transcendental idealism came to him in 1769, leading to his first statement of it the following year in his inaugural dissertation, On the Forms and Principles of the Sensible and Intelligible Worlds (1770). But it would then take him another decade, the so-called “silent decade”, to publish his full argument for transcendental idealism in the first edition of the Critique of Pure Reason, which appeared in 1781, and even then the relation between the epistemological idealism that he developed in that work and ontology continued to vex him: the first substantial review of the book in 1782 charged him with Berkeleianism, in other words, with the addition of ontological idealism to epistemological idealism, and Kant then tried to rebut that accusation in his attempted popularization of the Critique, the Prolegomena to Any Future Metaphysics of 1783, and to further defend that rebuttal of ontological idealism in the “Refutation of Idealism” that he added to the second edition of the Critique of Pure Reason in 1787. Even then he was not done with the subject, as we know from a dozen further drafts of the “Refutation” that he composed after that second edition of the Critique. Indeed, Kant continued to struggle with the clarification of his own position to the end of his life, attempting a restatement of transcendental idealism in the uncompleted material for a final book that has come down to us under the name of the Opus postumum. But since it was Kant’s presentations of his position in the two editions of the Critique and the Prolegomena that were most influential in his own time and have been since, we shall concentrate on those texts here.

Kant’s arguments for his transcendental idealism are distributed across all parts of his Critique of Pure Reason. He gives a direct argument for it in the Transcendental Aesthetic, supplemented by the Transcendental Analytic, and he gives an indirect argument for it in the Transcendental Dialectic by arguing that only his transcendental idealism can allow us to avoid the paradoxes or confusions of traditional metaphysics. We will comment first on Kant’s direct argument for transcendental idealism and then on his indirect argument for it through the critique of traditional metaphysics.

The direct argument is based on Kant’s claim, substantiated in the Transcendental Aesthetic, that we necessarily represent space and time and objects in them by means of our a priori representations of space and time, which are thus pure forms for the intuition of particular objects, and we can construct proofs of theorems about space and time by appeal to our a priori representations or in “pure intuition”. But how does this lead to idealism? Kant’s chief argument is that space and time can represent

no property at all of any things in themselves nor any relations of them to each other, i.e., no determination of them that attaches to objects themselves and that would remain even if one were to abstract from all subjective conditions of intuition,

and that space and time themselves can instead be only our a priori representations of them and the spatial and temporal features of objects in space and time only features of our representations of them or of the “appearances” of objects, because

neither absolute nor relative determinations can be intuited prior to the existence of the things to which they pertain, thus be intuited a priori. (A 26/B 42)

The decisive point of this argument is the following: although because of our forms of intuition our particular representations necessarily have spatio-temporal structure, any objects that had that structure independently of our so representing them would at best have such structure contingently, and thus the supposedly synthetic a priori propositions about space, time, and their mathematics would not be necessarily true throughout their domain. Kant makes this key point several times. In the Critique, he poses the rhetorical question,

If there did not lie in you a faculty for intuiting a priori; if this subjective condition were not at the same time the universal a priori condition under which alone the object of…intuition is possible; if the object ([e.g.,] the triangle) were something in itself without relation to your subject: then how could you say that what necessarily lies in your subjective conditions for constructing a triangle must also necessarily pertain to the triangle in itself. (A 48/B 65)

Similarly, in the Prolegomena he writes that

Pure mathematics, and especially pure geometry, can have objective reality only under the single condition that it refers merely to objects of the senses, with regard to which objects, however, the principle remains fixed, that our sensory representation is by no means a representation of things in themselves, but only of the way in which they appear to us,

for on the contrary supposition

it absolutely would not follow from the representation of [e.g.] space, a representation that serves a priori, with all the various properties of space, as foundation for the geometer, that all of this, together with what is deduced from it, must be exactly so in nature. The space of the geometer would [or could] be taken for mere fabrication and credited with no objective validity, because it is simply not to be seen how things would have to agree necessarily with the image that we form of them by ourselves and in advance. (§13, Note I, 4:287)

So, Kant concludes, in order to be necessarily true throughout their domain, the synthetic a priori propositions about space and time—and this includes not just the specific propositions of geometry or mathematics more generally but also the general propositions derived in the metaphysical expositions, such as that space and time are infinite singular wholes with parts rather than instances—must be true only of the representations on which we impose our own forms of intuition, and cannot be true of things as they are in themselves. This is Kant’s chief argument for epistemological idealism, the view that the way things appear to us essentially reflects our cognitive capacities rather anything intrinsic to them, combined with indeterminate ontological realism, the view that there are things independent of our representations of them but because our most fundamental ways of representing things cannot be true of them we cannot know anything about them other than this fact itself.

In a passage added to the second edition of the Critique, Kant also points out that by arguing for the “transcendental ideality” of spatio-temporality—that it is a necessary feature of our representations of things but not a feature of things as they are in themselves at all—he does not mean to degrade space to a “mere illusion”, as did “the good Berkeley” (B 71): his position is that it is a subjective but necessary feature of our way of representing things, similar to secondary qualities such as color or fragrance (B 70n) in being subjective but unlike them in being necessary (see also A 29/B 45), and he thinks that by failing to see that the spatiality (in particular) of our representations is necessary, Berkeley has unnecessarily “demoted” it to a mere illusion. Kant’s larger objection to the charge that his position is not different from Berkeley’s is, however, that while denying the spatiality and temporality of things as they are in themselves, he has provided no reason to deny that there are things distinct from our representations of them and our own minds as representing them. But since this larger objection is most clearly expounded and defended in the Prolegomena and the “Refutation of Idealism” added to the second edition of the Critique, which is inserted into the Transcendental Analytic, discussion of it can be deferred for now.

Kant does not need to mount a separate argument for transcendental idealism in the Transcendental Analytic, because while that is aimed at showing that the use of certain concepts (the categories of pure understanding) and principles (the principles of pure understanding) are necessary conditions of any cognition of objects at all, indeed of self-consciousness (apperception) itself, but also yield knowledge only when applied to intuitions, pure intuitions in the case of pure mathematical cognition and empirical intuitions in the case of everything else (thus Kant’s famous statement “Without sensibility no object would be given to us, and without understanding none would be thought. Thoughts without content are empty, intuitions without concepts are blind”; A 51/B 75), since empirical intuitions have already been shown to yield appearances rather than things in themselves, it automatically follows that the categories and principles of pure understanding will also yield cognition only of appearances. Nevertheless, Kant reaffirms transcendental idealism during the course of the Transcendental Analytic.

The Transcendental Dialectic, the second half of the Critique of Pure Reason in which Kant provides the critique of traditional metaphysics is explicitly intended to give an indirect proof of transcendental idealism (B xx). Specifically, the middle section of the Dialectic, entitled “The Antinomy of Pure Reason”, is supposed to provide this indirect proof. All three sections of the Dialectic, thus the preceding “Paralogisms of Pure Reason” and the following “Ideal of Pure Reason”, are supposed to show that the faculty of reason’s inevitable conception of the “unconditioned”, that which is a condition for everything else but itself has no condition, can never provide knowledge of any object because knowledge requires intuition as well as concept, and intuition is always conditioned—the representation of any region of space is conditioned by more surrounding it, and that of any region of time is likewise conditioned by the representation of more time before and after it. Reason can form “transcendental ideas”, more properly, “transcendent concepts” (A 327/B 384), that is, the ideas of an unconditioned subject (the self as substance), an unconditioned whole of all things and events (a completed world-whole), and an unconditioned ground of all possibility (God) (A 334/B 393). These ideas, according to Kant, may be useful as guidelines for scientific research and even necessary for the purposes of practical reason, but they outrun the limits of intuition and therefore theoretical cognition. This general claim itself does not entail transcendental idealism, that is, it does not identify space and time with our own forms of intuition. However, Kant’s claim is that the paradoxes diagnosed in the “Antinomy of Pure Reason” can only be resolved on the basis of transcendental idealism. In the case of the first two antinomies he argues that both sides essentially concern space and time or the things in them, and that since space and time as forms of intuition are indefinitely extendable and divisible, both sides of the debates, the theses and the antitheses, are false: space and time and thus the totality of things and events in them (the world) are neither bounded and finite or unbounded and infinite but indefinite (even though particular things within space or periods within time may have determinate boundaries). In the case of the third and fourth antinomies, however, Kant argues that the distinction between appearances and things in themselves that is at the heart of transcendental idealism makes it possible for both sides to be considered true, since they concern different objects: in the empirical world of experience, there are only ever indefinitely extending chains of causes and effects, each moment of which is necessary relative to its causal laws (the third antithesis) but contingent because no antecedent cause is absolutely necessary or necessary considered in itself, but outside of the empirical world there is nothing to prevent there being an absolutely necessary thing in itself (God) nor acts of absolute spontaneity on the part of that absolutely necessary being or even lesser beings, such as finite agents. Again, Kant’s claim will ultimately be that we have necessary and sufficient practical grounds for affirming both our own freedom and the existence of God, but these do not yield theoretical cognition (B xxx).

Kant’s antinomies led to the dialectical methods of Fichte, Schelling, and Hegel, and were thus to prove immensely influential. But it was clearly controversial whether the antinomies in fact required the distinction between appearances and things in themselves; Hegel, for example, surely thought not. For the argument that only transcendental idealism can resolve the antinomies seems to be circular: unless one assumes that our representations of space and time give us not only reliable but also complete information about the nature of space and time and all things in them, there is no reason to assume that the limits of our representations of space and time—their indefiniteness and the contingency of any starting- or stopping-point in them—are also in fact true of space and time and everything in them in themselves. Kant’s indirect proof for transcendental idealism therefore is not conclusive.

Kant himself did not think so, of course. He was utterly committed to transcendental idealism. When confronted with the challenge that transcendental idealism was nothing but Berkeleianism, however, that is, the reduction of all reality to ideas and the minds that have them, he recoiled. This objection was made in the first substantial review of the first edition of the Critique, written from an empiricist point of view by Christian Garve and then redacted by J.H. Feder in 1782 (in Sassen 2000). Kant defended himself by a more precise formulation of his doctrine in the Prolegomena (1783) and further by the insertion of a “Refutation of Idealism”, specifically “material idealism”, into the Transcendental Analytic in the second edition of the Critique (1787). Kant’s claim in the Prolegomena is that his position should be called “formal” or “critical” idealism rather than “material” idealism because it merely identifies space and time with our forms of intuition but does not otherwise deny the reality of the objects in space and time. As he puts it:

There are things given to us as objects of our senses existing outside us, yet we know nothing of them as they may be in themselves, but are acquainted only with their appearances, i.e., with the representations that they produce in us because they affect our senses. Accordingly, I by all means avow that there are bodies outside us, i.e., things which, though completely unknown to us as to what they may be in themselves, we know through the representations which their influence on our sensibility provides for us, and to which we give the name of a body—which word therefore merely signifies the appearance of this object that is unknown to us but is nonetheless real. Can this be called idealism? It is the very opposite of it. (4:288–9)

At this stage, Kant’s response to the identification of his position with traditional ontological idealism is basically incredulity: he cannot understand it because in his view he has only given reasons for removing space and time from things to our representations of them, just as earlier philosophers had given (different) reasons for relocating properties like color from object to subject, but has provided no arguments against the existence of those things themselves, which he, like any other sane person, takes for granted.

By the time of the second edition of the Critique, however, Kant must have come to see the need for a positive defense of the assumption of the existence of things in themselves that ground our spatio-temporal representations of body (although, since those things in themselves are not supposed to be spatio-temporal and causality is supposed to be a spatio-temporal relation, they cannot precisely be said to cause our spatio-temporal representations). Kant’s argument—which in the following years Kant would attempt to improve a dozen times (see Guyer 1983 and Kant 2005)—is that we can only achieve “empirically determined consciousness” of our own existence, or a determinate temporal ordering of our own representations, by correlating them with something enduring outside of and distinct from them:

The perception of this persistent thing is possible only through a thing outside me and not through the mere representation of a thing outside me. (B 275)

Spatiality may be acknowledged to be only my way of representing things outside me, but insofar as anything in space is used to determine the order of my own representations it must be regarded as being ontologically distinct from my representations of it even if its phenomenology is subjective, that is, even if spatiality is only our way of representing ontological independence (see A 22/B 37). In this way he proves, contra Berkeley who denies it and Descartes who doubts it, that our phenomenologically spatial representations are “grounded” in something ontologically distinct from those representations. Kant’s “Refutation” was intended precisely to demonstrate that epistemological idealism, the argument that our most basic forms of knowledge in fact reflect only our own forms of intuition and conceptualization, could and must be combined with indeterminate ontological realism.

It may well be asked of Kant’s “Refutation of Idealism”, as it had already been asked of his yet-to-be-named transcendental idealism in 1770 by such distinguished contemporaries as Johann Heinrich Lambert, Johann Georg Sulzer, and Moses Mendelssohn, whether it is really compatible with the transcendental ideality of time, that is, whether it does not presuppose the reality of the temporality of the enduring object it proposes by means of which to determine the sequence of our own representations as well as of the self that has that sequence of representations (isn’t the sequence of representations, they essentially asked, really a sequence?). But we will not further pursue that question here, because all of Kant’s successors were more concerned with the viability of Kant’s general distinction between appearances and things in themselves rather than with the specifics of Kant’s argument for transcendental idealism from a priori knowledge or of Kant’s proof that we can assert the existence of things in themselves in spite of that distinction. This concern began with the famous objection of F.H. Jacobi, made in the appendix to his 1787 book on David Hume, that without the assumption of things in themselves he could not enter into the critical system, but that with it he could not remain within the system; that is, he felt that 