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C. S. Lewis’s Dangerous Idea: A Philosophical Defense of Lewis’s Argument from Reason

Victor Reppert
Published by IVP Academic in 2003

Like many of us, Victor Reppert, Professor of Philosophy at Glendale Community College in Arizona, has long been intrigued by the thought of C. S. Lewis, especially his so-called argument from reason (developed by Lewis most fully in chapter three of his book Miracles: A Preliminary Study). C. S. Lewis’s Dangerous Idea, an obvious allusion to Daniel Dennett’s Darwin’s Dangerous Idea (1995), is a title doubtlessly chosen to highlight Lewis’s anticipation of the Darwinian naturalists’ repackaging of age-old arguments. The author of numerous articles on Lewis’s arguments, Reppert provides his readers a fresh, clear, and able exposition and defense of what he calls C. S. Lewis’s dangerous idea: that a purely naturalistic account of the world cannot explain the reality of human rationality.

In the first of six chapters, Reppert tackles the unfortunate and misguided tendency of many to dismiss Lewis’s arguments as less than serious. This, he observes, is often a result of “the personal heresy,” that is, “focusing on Lewis himself rather than what he had to say”(28). For example, given his preoccupation with other matters, Lewis did not offer detailed responses to many of the major philosophical problems of his day (such as absolute idealism and logical positivism), thus costing himself—in the eyes of many, at least—the status of “real philosopher” (20), never mind the success of his arguments. This sentiment is short-sighted indeed; Lewis’s thought, Reppert argues, deserves fair and honest consideration.

Before approaching Lewis’s argument from reason (AFR), Reppert spends the second chapter discussing how we are to go about assessing apologetic arguments. He frames the discussion in terms of three views on the relationship between faith and reason: fideism, strong rationalism, and critical rationalism. It is fairly obvious, given his eagerness to discuss and defend Christianity’s claims, that Lewis was anything but a fideist. Accordingly, Reppert makes quick work of this stance. At the opposite extreme is strong rationalism, “the view that in order for a religious belief system to be properly and rationally accepted, it must be possible to prove that the belief system is true” (31). This is a view strongly reminiscent of W.K. Clifford’s principle that “it is wrong always, everywhere, and for anyone, to believe anything upon insufficient evidence.”1 Reppert urges the realization, which he identifies implicitly in Lewis’s Miracles, that people have both rational and nonrational motivations for coming to their beliefs. This leaves critical rationalism, the more moderate stance that “religious belief systems can and must be rationally…evaluated, although conclusive proof of such a system is impossible” (36). Lewis, Reppert concludes, is best read as a critical rationalist, which reveals a good deal about how Lewis’s AFR should be taken.

The third and longest chapter of the book unpacks the argument itself. The crux of the argument is that a naturalistic worldview does not possess the resources needed to explain humans’ ability to draw rational inferences, that is, to reason. At base, naturalistic explanations must appeal to the blind, mechanistic, nonpurposive operations of nature; naturalists have no recourse to purposive explanations for rationality (which, Reppert observes, seems to suit naturalists like Dennett just fine). The alternative, of course, is that we are created in the imago dei, which Christians have long understood to include rational intellect (see, for example, Thomas Aquinas, Summa Theologica Ia q. 93 a. 4 or The Westminster Confession, chapter 4). Of course, both Lewis and Reppert affirm the theistic account.

Reppert begins with the original version of Lewis’s AFR: simply put, that materialism (the naturalistic view that the basic substances of the physical world are bits of matter [47]) entails that all thoughts be fully explainable as the result of irrational causes, thus rendering all thoughts invalid, including the thought ‘materialism is true’—in other words, naturalism is self-contradictory (54). A pretty argument too, though not without flaws. On February 2, 1948, renowned philosopher Elizabeth Anscombe and Lewis had a now-famous exchange on the latter ’s AFR at the Oxford Socratic Club—an exchange the substance of which has, unfortunately, been misunderstood greatly. And here is where Reppert shine as both philosopher and biographer: in addition to articulating clearly the details of Anscombe’s criticisms as well as the revised version of Lewis’s AFR, Reppert rebuts the “Anscombe legend,” that is, the perception of some that his exchange with Anscombe dealt Lewis a crushing psychological blow, causing him to abandon the apologetic enterprise (it is telling that Anscombe herself, in the introduction to volume two of her Metaphysics and the Philosophy of Mind [1981], described such “odd accounts” as “an interesting example of the phenomenon called ‘projection’”).

Having thus shown that Lewis’s argument was not, in fact, permanently refuted, Reppert takes up the task of making a positive case for the incompatibility of naturalism and rational inference. He begins in the fourth chapter by examining seven formulations of the AFR, each appealing to different features of reality to debunk naturalism and accept theism. Interestingly, these features of epistemic reality (such as intentionality, truth, and consciousness) are necessary to both naturalists and theists alike (on pain of irrationality), but as Reppert shows, they are incompatible with naturalism. This implies the necessity of an “explanatory dualism,” which is simply the affirmation that “while some events in nature can be explained in terms of purely mechanistic causes, the elements of rational inference…cannot” (87).

Developing this explanatory dualism, Reppert spends the fifth chapter fielding popular objections to the AFR. Of particular interest here is the undeniable reality of “aboutness” in our thoughts and the very reliability of our rational faculties, both of which highlight the need for explanatory dualism. The former appears to be a basic fact of experience, not reducible to materialist terms (as Lewis observes, “to talk of one bit of matter being true about another bit of matter seems to me to be nonsense”2). The latter, the realization that, given naturalistic evolution, we have no reason to rely on our belief-producing mechanisms. Each of these realities demands a purposive explanation. Since materialism lacks the non-physical resources needed to account for such realities, he concludes, explanatory dualism takes the day.

If successful, Reppert has defended the AFR from its detractors thus far and has shown that the fundamental fact of the universe must be rational, a fact for which naturalism cannot account and for which theism can. Nevertheless, he acknowledges that “physicalism remains the prevailing paradigm in the philosophy of mind” (105). But why, if, as has been shown, naturalism cannot account for rationality, is this the case? Reppert attributes this to the so-called “inadequacy objection,” which he takes up in the final chapter. The “objection” is really “…a prevailing conviction that when the best naturalistic resources available are employed to produce an understanding of the world and mysteries are left over, we do little or nothing to explain those mysteries by invoking ‘souls’ or by explaining them in terms of God” (106).

Supernatural explanations, at least according to outspoken atheist Keith Parsons, really do not explain anything, since genuine understanding comes only from physical explanations—a fantastically question-begging claim which Reppert sees for what it is. Besides, “explaining reason in terms of the inherent rationality of God is no more question-begging than explaining physical states in terms of prior physical states” (122). The “inadequacy objection,” Reppert concludes, is likely a result of “scientific fideism,” that is, the padlocking of materialism against even the possibility that it might be in error (126).

C. S. Lewis’s Dangerous Idea is a fine work. Well-written, the book glows with Reppert’s engaging style and ability as an incisive thinker. Though at times demanding, it is accessible to a broad audience. This book represents a real advance in apologetics generally and Lewis scholarship specifically, and I highly recommend it to anyone interested in either.

Cite this article
R. Keith Loftin, “C.S. Lewis’s Dangerous Idea: A Philosophical Defense of Lewis’s Argument from Reason”, Christian Scholar’s Review, 37:3 , 389-391


  1. W. K. Clifford, “The Ethics of Belief,” in Lectures and Essays (London: Macmillan, 1879). See also Peter vanInwagen, “Quam Dilecta,” in God and the Philosophers, ed. Thomas V. Morris (New York: Oxford UniversityPress, 1994).
  2. C. S. Lewis, Christian Reflections (Grand Rapids, MI: Eerdmans, 1967), 64.

R. Keith Loftin

R. Keith Loftin, Philosophy (Graduate Student), Louisiana State University