In his famous 2006 address at the University of Regensburg, Pope Benedict XVI spoke of the importance of academic inquiry into the “reasonableness of faith.” The Pope noted that a former colleague had quipped that the university had two theological faculties, one Catholic, the other Protestant, devoted to something that did not exist: God. The Pope also observed that even—perhaps especially—in an age of radical skepticism, it was “necessary and reasonable to raise the question of God through the use of reason, and to do so in the context of the tradition of the Christian faith.”

In our culture, which embraces skepticism as the default position in matters philosophical, it may seem strange to speak of the reasonableness of faith or to claim that God’s existence can be demonstrated. We are presented with a choice between a secular reductionism that excludes any appeal to the supernatural and God, on the one hand, and a religious reductionism that rejects any role for reason as a criterion of truth in matters of faith, on the other hand.

Contemporary rejections of proofs for the existence of God fall into three broad categories: (1) various forms of positivism and scientism that hold that science disproves the existence of God; (2) philosophical arguments that challenge either the possibility of conclusive demonstrations of any kind, or, more specifically, the possibility of proofs for a transcendent cause; (3) theological objections to the possibility of rational demonstrations of God’s existence. Although the so-called “new atheism” associated with the natural sciences is the best known of the three, its arguments are by far the least sophisticated.

Matthew Levering’s new book, Proofs of God: Classical Arguments from Tertullian to Barth, concerns primarily the second and third categories. He offers a survey of the views of twenty-one thinkers on whether or not human reason can demonstrate the existence of God. Levering provides summaries of arguments, essentially in the Christian tradition, from three historical epochs: the Patristic and Medieval Eras, the Reformation and Enlightenment, and the nineteenth and twentieth centuries. With extensive footnotes, brief biographical entries, and a good bibliography, the book is, as Levering says, a “textbook” that can be consulted selectively.

Can There Be a Return to Traditional Metaphysics?

One of Levering’s goals is to contribute to a “metaphysical retrieval” of the traditional approach of philosophers such as Thomas Aquinas who think that reason is able to prove the existence of God. Since Levering deals with complex philosophical and theological questions, his book is most appropriate for those familiar with the history of philosophy and Christian theology.

Referring to Catholic teaching, Levering points to the famous text of the First Vatican Council (1870): “God, the source and end of all things, can be known with certainty from the consideration of created things, by the natural power of human reason.” Levering, citing the analysis of Thomas Joseph White, argues for the importance for believers of an inquiry into rational demonstrations of God’s existence: “Far from threatening to make divine revelation redundant, the demonstrability of God’s existence makes revelation all the more desirable and urgent.”

For those who believe that God exists, there are two dangers to be avoided: thinking that reason cannot prove the existence of God and thinking that proposed demonstrations for the existence of God somehow limit God to the categories of human understanding. Here, it is important to remember Thomas Aquinas’s famous distinction between knowing that something is and knowing what it is (i.e., its essence). For Thomas, reason can reach the conclusion that God is, but can only say things in a kind of negative way about who God is (that God is not like creatures and does not possess the characteristics that creatures possess). As Levering notes, what the traditional proofs of God show is that “a transcendent, infinite source of all finite things exists.”

The proofs that Levering defends depend upon a recognition of the radical contingency of finite things, on the principle of causality that allows us to reason from finite effects to an infinite, transcendent cause, and, more generally, on a realist epistemology that affirms that we are able to have knowledge of the world as it is. The challenges to these claims are especially evident in the thought of David Hume and Immanuel Kant, and Levering’s analysis of their philosophical positions is a key feature of the project he has undertaken. In many ways, recent thinkers such as Heidegger and Wittgenstein operate within a widely accepted Humean and Kantian view of knowledge and its limits. This philosophical heritage also informs the views of some Christian theologians who, accepting Hume’s critique of real causal interconnections, conclude that there are no proofs of God’s existence.

Hume, Kant, and Modernity

The increasing interest in skepticism in early modern Europe—a reaction both to late medieval philosophical traditions associated with an overemphasis on divine will and to the controversies of the Reformation about the criteria for religious truth—provided the context for David Hume’s critique of any demonstration for the existence of God. His conclusion that God’s existence cannot be proven is part of Hume’s broader claim that we “can know only discrete facts but not universal necessities,” including any real causal dependence in nature. For Hume, we cannot be assured “from the existence or action of one object, that it was followed or preceded by any other existence or action.” Any assurance for our thinking would require that we are able to discover a necessary relationship between what we call causes and effects. As he says, simply noting that two events exhibit a relation of “contiguity and succession” is not sufficient to establish a necessary causal connection between them.

The criticism Hume makes is first of all an epistemological claim. As Levering observes, since “Hume eschews inquiry into being, he is only willing to consider how the idea of cause relates to the idea of effect. He is therefore unable to see how the two are co-implicated in one event.” In denying that the principle of causality describes a fundamental feature of reality, Hume rejects a key element of classical demonstrations for the existence of God. But the causality that Hume identifies as being only our way of describing individual experiences of contiguity and succession is not the conception of causality that informs the classical approach of thinkers such as Thomas Aquinas. Temporal succession and contiguity are not fundamental characteristics of cause and effect which, understood in the light of actuality and potentiality (the cause’s making actual that which is only potential), are features of a single reality. For Thomas, the principle of causality captures the metaphysical truth of real dependence in nature, real dependence in the order of being. Of course, to speak of the “order of being” or of “metaphysical truth” presupposes a philosophy that sees as its goal the grasping of what is true about the world: a truth that in its most profound sense concerns what it means for things to be. Ultimately, to engage Hume’s critique requires an analysis of the adequacy of his empiricist philosophy, of the primacy he gives to epistemological questions, and of his denial of a realist metaphysics. At least we can recognize that what Hume calls the principle of causality is not the principle that informs classical demonstrations for the existence of God.

For Hume, “we assume that effects have causes because this is what our experience accustoms us to infer.” Immanuel Kant offers a more sophisticated challenge to proofs of God. For Kant, “cause” is an a priori concept among other a priori concepts. It is a form of thought like “substance,” “possibility,” “necessity,” and the like. The human mind uses these pure concepts—which exist prior to our experiences of the world—synthetically to organize our various sense perceptions. These concepts, including causality itself, do not refer to the way things are. For Kant, our experience of finite, contingent things is insufficient to allow us to appeal to some kind of “transcendent causality” that accounts for their existence. Kant’s critique relies upon “the view that knowledge never arrives at actual beings but is merely the interplay of concepts.” There is in this view a shift in the meaning of metaphysical reflection: no longer does it concern the order of the real but rather the immanent operations of human reason.

Kant is famous for his criticism of what has been called the ontological argument for the existence of God: the argument that begins with the idea of God as that than which nothing greater can be thought (only an a priori concept of a highest being), and concludes that there must exist such a necessary being. Thomas Aquinas also rejects the validity of an argument from the idea of an infinitely perfect being. Thomas’s arguments begin with finite, contingent things and then argue to the reality of an absolutely Self-Sufficient Being. Although Thomas’s proofs of God are not threatened by Kant’s criticism of any ontological argument, they do depend upon a metaphysics, philosophy of nature, and epistemology that Kant thinks are false. A response to Kant must include a defense not only of the claim that we really know the existence and natures of what we experience but also the claim that our notion of causality can extend beyond the phenomena of immediate sense experience: that we can argue from finite effects to infinite cause.

Levering helps us to see that any retrieval of traditional metaphysics and proofs of God requires a response not only to Hume and Kant but also to Heidegger’s criticism of “onto-theology,” according to which “God” is only the highest being, and hence not a truly transcendent cause. One must respond as well to Wittgenstein’s claim that the question of God’s existence is nonsensical because “there is no possibility for our language to speak in any way about the transcendent.” Hence, as Levering observes, “the demonstrations of God’s existence are . . . cut off before they can begin, since without real knowledge of being (act/potency, analogous modes of being) there can be no sense to the principle of causality or the principle of non-contradiction.”

Resisting Fideism and Emotivism

If one believes that God exists, and one accepts the Kantian and Heideggerian critiques, there is a strong temptation to embrace a theological view that God’s existence is the subject only of revelation and faith. Another temptation is to base claims for God’s existence exclusively on some inner experience, as though human subjectivity offers the best road to respond to skepticism and to the challenges of modern philosophy. A retrieval and defense of Thomistic metaphysics would speak to the inadequacy of these theological conclusions.

Levering thinks that much of modern philosophical discourse misunderstands classical philosophy and that any retrieval of classical philosophy and its proofs for the existence of God needs to begin with a clear account of the metaphysical realism of thinkers such as Thomas Aquinas. A defense of this realism requires the recognition that a fundamental object of human knowledge is the being of the things we experience in the world. The first principles of our knowing cannot themselves be demonstrated; and rationality itself, not just demonstrations for the existence of God, depends upon them. The defense of philosophical realism—that there is an external world and we can come to know it—begins with dialectical discourse, since its principles are the starting points for all argumentation. An important task in such discourse is to make sure we are talking about the same thing in the same respect. Levering’s discussion of how terms such as “cause,” “substance,” and “being” do not mean the same thing in modern thought as they do in classical philosophy can serve as a way to begin to retrieve classical philosophy.

The metaphysical and epistemological presuppositions that inform various arguments that deny we can demonstrate the existence of God—just like the presuppositions that affirm that we can—are just that: presuppositions. Only after a careful exposition of these principles can we evaluate whether they are true. Whether there can be such an evaluation is itself a matter of contention that needs to be addressed. What is at issue, finally, is whether or not things actually exist; whether we encounter and come to know a world of existing beings, and what it means to speak of existing things precisely as existing. The contingency of finite beings is not fundamentally that they could be other than they are, but that they might not exist at all. The world of finite, contingent creatures is just that, a world of creatures, and to be a creature necessarily means to depend upon a continuing transcendent cause of existence.

In a dialectical mode, Levering puts the point starkly: “Can we really think of the being of a rock, for example, as only a conceptual predicate or property and not as actual (contingent) being? Surely not.” Saying this, of course, does not make it so, but Levering’s book shows us the road that needs to be taken and the dead ends that need to be avoided as we think deeply about what it means for things to be.