And they ask you about the spirit—Say: The spirit is from the affair of my Lord.

– Qur’an 17:85

Human nature has been a central theme of intellectual and mystical contemplation from ancient times and across civilizations. By virtue of distinctive traits like rationality, knowledge, speech, and moral agency, man was considered a world apart from the rest of nature, and the spirit of man was recognized as something unique and wondrous. In Islam, knowledge of one’s soul is the point of departure for its purification and for attaining divine love, given that an authentic life of piety and altruism emanates from a sanctified soul absorbed in the remembrance of God.

With the advent of modernity, new philosophical commitments brought new terminology and novel theories. What most civilizations traditionally named spirit or soul is often called “mind” today due to the theological implications of the classical terms.1 The branch of modern philosophy that deals with what the medievals called soul is the philosophy of mind, a field of inquiry that relates to many critical issues in both academic and public discourse. For example, is transhumanism—the theory that through science and technology, humans can evolve beyond current physical limitations and elude aging or even death—a genuine possibility if there is such thing as a human soul? Is atheism, which attempts to explain all of reality through materialism, rendered incoherent if man has an immaterial intellect? Do human consciousness, thought, and rationality signify the existence of the soul? With the scientific method as its governing approach to knowledge, modernity has attempted to unpack man’s inner mystery through a variety of somewhat contrasting paradigms, such as neuroscience, Freudian psychoanalysis, behavioral genetics, and evolutionary psychology.2 So much of human culture, society, and even politics hinges on our awareness of ourselves as human beings and our relationship with the world we inhabit. How we understand the mind is central to that awareness.

Historically, Muslim theologians generally espoused an integrative substance dualism of body and soul. As Syed Naquib al-Attas defines, “Man has a dual nature, he is both body and soul, he is at once physical being and spirit.”3 While Muslim scholastics deemed the essence of the soul to be a mysterious divine secret—what the mystic Ibn ʿAjībah (d. 1224/1809) termed “a luminous, lordly subtlety”4—it was evident that the soul is (a) distinct from the body, though deeply integrated with it, and (b) the locus of human consciousness. Yet as spiritual substance, the soul is inaccessible to empirical investigation: consciousness is located in a realm beyond the physical—not in mulk, the corporeal world, but in malakūt, the spiritual world. The question of its artificial replication is for most theists a nonstarter, since human manipulation is limited to the physical domain. The possibility of replicating consciousness (or generally, any feature of the mind) largely emerges from what is termed naturalism—the view that all things and events in nature can be explained physically, even if the hard sciences have yet to discover their explanations, because natural processes take place of their own accord. As an explanatory idea, naturalism is most often associated with materialism—or physicalism—an ontological position which holds that only “physical matter” really exists.5

In Death of the Soul: From Descartes to the Computer, William Barrett has traced the gradual exclusion of mind from intellectual deliberations on reality.6 The seventeenth century inaugurated a new science that viewed the world as a machine, based on a theory of matter that deems physical objects to be composites of particles in empty space. The mechanism of this Newtonian science was coupled with John Locke’s famous distinction between primary and secondary qualities. According to Locke, since physical objects are merely quantifiable aggregates of molecules, they have only “primary qualities” like extension and shape, while “secondary qualities” like color, taste, or sound are absent from the objects-in-themselves and exist only as sensations in people.7 What is “out there” is only the quantitative and measurable. Unsurprisingly, if quality is generally removed from one’s account of what actually exists, then the mind with all of its qualitative features is also susceptible to being reduced to the quantitative brain/body.

The eighteenth century brought another watershed moment with the skepticism of David Hume, who reduced human experience to a succession of sense impressions, and moreover considered the “self” to be nothing more than a bundle of perceptions. Barrett laments, “The I, or ego, suffered here a blow from which the fragmentation of the Modern Age has never rescued it. We live in a world where the flow of sensations, copiously fed to us by all the devices of technology, can virtually turn the ordinary citizen into a heap of perceptions.”8 Barrett notes that Hume’s categorical mistake, though, was to search for the self in objective sense-data rather than to recognize his own subjectivity in that search. Yet subsequent thinkers were captivated by Hume’s ideas; and as technology and the hard sciences rapidly advanced, materialism would emerge as a reigning paradigm for modern science. Nevertheless, in the eyes of many philosophers of mind, materialism has now reached an insurmountable quandary in the question of consciousness.

Physicalist Theories About the Mind

Physicalist theories that attempt to explain mental states include eliminative materialism, behaviorism, identity theory, and functionalism.9

In light of the continued success and explanatory power of modern physics, physiology, and neuroscience, eliminative materialism [EM] denies the existence of psychological states (sensations, thoughts, feelings, etc.): notions like “John is in pain” and “Fred enjoys vanilla ice cream” are eliminated and replaced by “John’s brain is in neural state X” and “Fred’s brain is in neural state Y.” For eliminativists like Paul Churchland, the commonsense view that mental states are real—what he terms “folk psychology”—is simply a theory, and one that is devoid of explanatory power. So he argues that it is a false theory: its history of failure to provide scientifically useful explanations leads to the conclusion that the mental states of folk psychology are merely illusions. However, opponents of this view argue that (a) our psychological states do not themselves comprise a theory but require a theory (or metaphysical worldview) to explain them; and (b) EM as a theory proves incoherent and self-refuting, insofar as its claims that it is true, and that folk psychology is false, reveal intentionality, itself a profoundly salient state of the mind.10 Truth claims are propositional attitudes that EM denies. Acceptance of EM presupposes folk psychology, since EM rejects notions like “acceptance.”

Another materialist/physicalist theory of mind is philosophical behaviorism [PB], according to which psychological states are logically equivalent to “dispositions” of behavior: pain is not a subjective reality, but is only the tendency to wince or cry or say “Ouch!,” etc. To justify PB, proponents adduce as evidence the strong connection between mental states and behavior, which for them can readily be explained as a connection between behavioral dispositions and behavior. To be “in pain” is to be “disposed” to certain behaviors (crying, wincing, …); to be “happy” is to be “disposed” to certain other behaviors (smiling, laughing, …); and so forth.

PB is also supported by the Vienna Circle’s theory of verificationism in the philosophy of language, a theory that was central to the Circle’s broader philosophy of logical positivism, a form of empiricism that denied the possibility of metaphysics. This early twentieth-century group of philosophers and scientists in Vienna, Austria, argued that the meaning of any statement is rooted in its method of verification, and verification was limited to sense-data: if a statement could not be verified empirically, it did not have rational (or “cognitive”) meaning. Thus, some PB proponents argued that if sensory observation is the only avenue of ascertaining the meaning of a proposition, then private mental states can be translated to observable behavior without losing meaning. Most philosophers, though, do not subscribe to verificationism, which has been almost unanimously discarded in the philosophy of language for several reasons, such as the theory’s incoherence—verificationism itself cannot be empirically verified.11Moreover, critics of PB point out that PB is demonstrably flawed, for it is conceivable that a person could have rich and changing mental states yet refrain from any behavior at all.12 Not all pain is expressed through crying or wincing, and not all happiness through smiling and laughing. And quite often, a person’s behavior is informed by innumerable mental states (beliefs, emotions, desires, motives, …) that are near impossible to reduce to algorithms of corresponding physical behavior.