The Argument from Reason for the Existence of God – Reasonable Christianity

Scientific inquiry proceeds with the presupposition that nature is an orderly structure which is intelligible to the human mind. How is this coherent interaction between the human mind and the natural order possible? What is the origin and nature of human reason? For simplicity, I shall just focus on two dominant paradigms addressing these questions:

1) Naturalism and reductive materialism: the universe of space-time and all its interlocking processes exists as a causally closed continuum and nothing else exist. This being the case, reason and mental processes are merely some aspect of physical processes or neural activity.

2) Theism –The observable space-time is a contingent order produced and sustained by a necessary being called God. Indeed, this space-time framework may not even be the only order of reality created by God. Reason is derived from some form of supernatural or divine intervention.

C.S. Lewis’ Argument from Reason sets out to out to demonstrate why naturalism fails to account for the origin and reliability of reason. On the other hand, reason is better accounted for within a theistic framework.

The argument may be summarized as follows:

1) If naturalism is true, it must be able to explain everything that happens in terms of the closed system called nature.

2) But reason (mental phenomenon) cannot be explained within this closed framework.

3) Therefore, naturalism is not true.


How does C.S. Lewis defend premise (2)?

He begins by making a distinction between two forms of relations:

The “Cause and Effect” relation: Example – “Grandfather is ill because he ate lobster yesterday.”

The “Ground and Consequent” relation: Example – “Grandfather must be ill today because he hasn’t gotten up yet although he is an early riser.”

The first relation describes a dynamic connection between events or “states of affairs”. The second relation describes a logical relation of entailment between beliefs or assertions.

Now imagine a series of thoughts A, B, C…One would say a reasoning process is valid if it is the case that the later thought is entailed by the earlier thought(s). Naturalism requires also that every event must have an earlier cause, but that this cause is a part of nature. A series of thoughts succeeds in yielding knowledge only it is connected by both relations of causation and entailment. Lewis writes, “in order for a train of thought to have any value, these two systems of connection [causation and entailment] must apply simultaneously to the same series of mental acts.” [M15]

We can rephrase the first premise according:

1. If Naturalism is true, then valid reasoning occurs only if one thought can both entail and cause another thought.

But Lewis rejects this premise, “We know by experience that a thought does not necessarily cause all, or even any, of the thoughts which logically stand to it as Consequents to Ground. We should be in a pretty pickle if we could never think ‘This is glass’ without drawing all the inferences which could be drawn. It is impossible to draw them all; quite often we draw none… One thought can cause another not by being, but by being seen to be, a ground for it.” [M16]

It is arguable that Lewis holds on to a causal theory of knowledge which says that a person knows a proposition “P” not only because he believes P, but that P also somehow brings the case for the person to hold the belief that P is true. But Lewis notes also that it is not so much that thought A causes thought B which then causes thought C within a self-contained causal chain. We arrive at thought C thought an act of reasoning or inference. An appropriate example can be found in the famous syllogism:

A) All humans are mortal

B) I am human

C) Therefore, I am mortal


I first reflect and accept thought A. Then I reflect and accept thought B. Finally, I conclude thought C. In other words, knowledge goes beyond immediate sensations and is inferred from these sensations through a valid process of inference. It is not only the case that recognition of a ground could be the cause of assent; that assent is rational when such was its case. Lewis arrives at the crucial second premise:

2. One thought can both entail and cause another thought only if the first thought can be known to entail the second.

Lewis upholds thought as both an event and an insight,

“Acts of thinking are no doubt events; but they are a very special sort of events. They are ‘about’ something other than themselves and can be true or false…Hence acts of inference can, and must, be considered in two different lights. On the one hand they are subjective events, items in somebody’s psychological history. On the other hand, they are insights into, or knowings of, something other than themselves.” [M 16-17]

(This brings to mind Kant’s famous dictum in his Critique of Pure Reason, “Thoughts without contents are empty, perceptions without conceptions are blind…. Understanding can perceive nothing, the senses can think nothing. Knowledge arises only from their united action.”)

Lewis presses home his case for thought being an inferential process or insight,

“What from the first point of view is the psychological transition from thought A to thought B, at some particular moment in some particular mind, is, from the thinker’s point of view a perception of an implication (if A, then B). When we are adopting the psychological point of view we may use the past tense. ‘B followed A in my thoughts.’ But when we assert the implication we always use the present—‘B follows from A’. If it ever ‘follows from’ in the logical sense, it does so always. And we cannot possibly reject the second point of view as a subjective illusion without discrediting all human knowledge.  For we can know nothing, beyond our own sensations at the moment unless the act of inference is the real insight that it claims to be (emphasis added).” [M17]

Here lies the Achilles Heel of naturalism – naturalism regards thought as the effect of a physical cause, it provides no warrant to consider anything that is not the direct result of a physical cause. As such, there is just no way to take into consideration, much less accept thought as also the consequent of a reasonable ground. This restriction renders naturalism self-defeating.

“A theory which explained everything else in the whole universe but which made it impossible to believe that our thinking was valid, would be utterly out of court. For that theory would itself have been reached by thinking, and if thinking is not valid that theory would, of course, be itself demolished. It would have destroyed its own credentials. It would be an argument which proved that no argument was sound-a proof that there are no such things as proofs-which is nonsense. Thus a strict materialism refutes itself for the reason given long ago by Professor Haldane: `If my mental processes are determined wholly by the motions of atoms in my brain, I have no reason to suppose that my beliefs are true … and hence I have no reason for supposing my brain to be composed of atoms.’” [M14]

One further problem that naturalism fails to surmount is how to explain the origin of reason. Lewis notes that for evolutionary theory, reason, sentience, and life itself are latecomers in nature after a long historical process. He explains,

“And of course, for the Naturalist, this process was not designed to produce a mental behaviour that can find truth. There was no Designer; and indeed, until there were thinkers, there was no truth or falsehood. The type of mental behaviour we now call rational thinking or inference must therefore have been ‘evolved’ by natural selection, by the gradual weeding out of types less fitted to survive. Once, then, our thoughts were not rational. That is, all our thoughts once were, as many of our thoughts still are, merely subjective events, not apprehensions of objective truth…Now natural selection could operate only by eliminating responses that were biologically hurtful and multiplying those which tended to survival. But it is not conceivable that any improvement of responses could ever turn them into acts of insight, or even remotely tend to do so. The relation between response and stimulus is utterly different from that between knowledge and the truth known.” [M18]

As Lewis notes, the evolutionary processes can only bring forth a sensation or stimuli-response complex which should not be equated with knowledge.  For example, a child would instinctively withdraw his hand when it touches a hot stove, but the response has no object or content. It is at most a ‘subjective event, not apprehension of objective truth. An additional element must be introduced for this event to achieve the status of knowledge, that is the act of knowing. As contemporary philosophers would say, an instinctive reaction is not an intentional state. Knowledge is always about some specific facts in someone’s thought. Knowledge requires the intentionality and rational inference.

Lewis’ quarrel with the naturalism or natural selection is not that it is not capable of producing useful behavior. After all, many predators that do not exhibit reason still survive pretty well in the bush. But the problem with naturalism is that it can only recognize a succession of experience or events. That is, it can only show us that A is followed by B, but it cannot show us that A must be followed by B. It cannot proceed with reason or rational inference. In this regard, naturalism is itself a contradiction, “Naturalism is a prime specimen of that towering speculation, discovered from practice and going far beyond experience, which is now being condemned. Nature is not an object that can be presented either to the senses or the imagination. It can be reached only by the most remote inferences.” [M22]

Speculation or not, it remains the fact that naturalism is unable to account for the origin of the act of knowing.


We may now formulate additional premises for the argument:

3) If Naturalism is true, then knowledge exists only if evolution is able to produce the capacity to reason for creatures which initially had no capacity to reason

4) But evolution could not produce the capacity to reason from creatures lacking such capacity


We can now bring together the whole argument by Lewis:

1. If Naturalism is true, then valid reasoning occurs only if one thought can both entail and cause another thought.

2. One thought can both entail and cause another thought only if the first thought can be known to entail the second.

3) If Naturalism is true, then knowledge exists only if evolution is able to produce the capacity to reason for creatures which initially had no capacity to reason.

4) But evolution could not produce the capacity to reason from creatures lacking such capacity.

5) If Naturalism is true, inferential knowledge does not exist (from 3 and 4)

6) Without inferential knowledge, there is no sequence or entailment of thought

7) Conclusion – Therefore, if Naturalism is true, then valid reasoning does not occur (from 1, 2, 5 and 6).

We can rephrase the above logical steps as an argument from reliable knowledge:

1. If naturalism is true, our faculties cannot produce reliable information about the world beyond immediate sense perception.

2. But our faculties do reliably reveal the knowledge of the world beyond sense perception. (Presupposition of rational inference.)

3. Therefore, naturalism is false.

Given the inadequacy of naturalism one should be open to consider the possibility that perhaps intentionality, reason and inference have a non-physical origin. For example, we note both physical objects like chemical brain processes and ink marks by themselves do not possess inherent meaning and intrinsic intentionality. Therefore, any intentionality would have to originate from something else, that is the mind. Since the mind does not derive its intentionality from the physical ink marks, and for that matter the mind does not get its intentionality from any other physical object (there is no indication that the mind is simply a passive instrument to receive meaning), one can only conclude that the mind has intrinsic intentionality and is therefore non-physical. [PM 117-153; 170-210]

Finally, Lewis begins with the incontrovertible fact that we possess reason, or rather we reason (verb). Reason should be taken as the starting point rather than as the (unsuccessfully demonstrated) outcome of natural selection or evolution.

“[The Theist] is not committed to the view that reason is a comparatively recent development moulded by a process of selection which can select only the biologically useful. For him, reason—the reason of God—is older than Nature, and from it the orderliness of Nature, which alone enables us to know her, is derived. For him, the human mind in the act of knowing is illuminated by the Divine reason. It is set free, in the measure required, from the huge nexus of non-rational causation; free from this to be determined by the truth known. And the preliminary processes within Nature which led up to this liberation, if there were any, were designed to do so.” [M22-23]

Indeed, reason is able to produce insight because it is not trapped within a closed causal chain of physical events. Lewis celebrates this act of knowing —the act, not of remembering that something was so in the past, but of ‘seeing’ that it must be so always and in any possible world can be called supernatural as it “cannot be merely the exhibition at a particular place and time of that total, and largely mindless, system of events called Nature. It must break sufficiently free from that universal chain in order to be determined by what it knows.” [M23]

Lewis suggests that reason is not only possibly but likely a result of supernatural intervention, “Reasoning doesn’t ‘happen to’ us: we do it. Every train of thought is accompanied by what Kant called ‘the I think” [M29] which is perhaps a pointer to the fact that human thought is God-kindled, that is, I am a creature to whom God has given reason.


We may conclude with a brief argument to prove the existence of God.

8) Reason exists, but does not arise spontaneously from non-rational material.

9) Reason (rational human) began to exist at some point in time (evidence – that humans have the capacity to reason, assess truth claims and be convinced by arguments).

10) If knowledge/reason exists and Naturalism is false, then there is a supernatural source of all knowledge/reason.

11) These historically contingent rational humans must have derived their rationality from a prior (necessary) rational being.

10) Therefore, there is a supernatural, eternal, self-existent, rational Being who is the ultimate source of all knowledge.





C.S. Lewis. Miracles: A Preliminary Study HarperCollins 1998. [M]

John Beversluis. C.S. Lewis and the Search for Rational Religion. Eerdmans 1985.

Victor Reppert. C.S. Lewis’s Dangerous Idea: In Defense of the Argument From Reason. IVP 2003.

Edward Feser. Philosophy of Mind. One World Press 2006. [PM]

Erik J. Wielenberg. God and the Reach of Reason: C.S. Lewis, David Hume, and Bertrand Russell. Cambridge UP 2008.


Future discussions will include similar views by non-theistic philosophers like Thomas Nagel and Colin McGinn and the theistic philosopher, Alvin Plantinga. A final article on this theme will present the Transcendental Argument for God.


  1. Henry Hock Guan Teh says:

    Thumbs up for Kam Weng! Great write-up. Christianity is indeed a reasonable faith. Even Jesus utilized all the basic laws of rational and reasoning processes e.g. laws of identity,noncontradiction, and excluded middle. Jesus employed methods of argument like categorical, hypothetical, and disjunctive syllogisms. Besides that, Jesus also knew how to use reductio ad abdurdum, a fortiori, escape the horns of a dilemma argument. Such logic and reasoning employed by Jesus because He knows any transformation of a true Christian begins with the mind which thereon led to his or her commitment of the heart. Whenever the Holy Spirit transform the heart of an individual, He never by-pass the mind. No where in the Bible encourages the Christians to exercise their faith or love of God without the mind. It always the heart and the mind …. then is up to your free will to decide and accept. Otherwise, we are unreasonable Christians with an unreasonable faith.

  2. Dec says:

    Hm, I thought the AFR is mainly an argument against materialism or physicalism of the mind in favor of a sort of dualist view on consciousness. Aren’t philosophers such as Nagel or David Chalmers self-declared atheists who are not materialists at the same time?

    BTW, will you be writing on Plantinga’s evolutionary argument against naturalism (EAAN) in the future?

  3. Kam Weng says:

    Nagel and Chalmers are certainly dualists in the sense they reject the materialist’s claim to be able to explain the world purely on physical terms. Should also note there are different kinds of dualism, e.g. physicalism which sees an identity between consciousness and the brain and dual aspect theory which considers them as different aspects of the same thing. Perhaps it is not too wrong to put Nagel and Chalmers under the oxymoron term of ‘naturalistic dualism’.

    Keep in mind that even when while we critique naturalism/materialism, we need not be forced into the dilemma of choosing one or the other. Maybe should consider a ‘third-way’ – Aristotelian or Thomistic/Aquinas hylomorphism. More of this in the future?

    Nagel’s and Chalmers views are relevant precisely because they are non-theists. The materialist vetoes the possibility of a reality beyond nature and insinuates that Christians like C.S. Lewis reject materialism and seek refuge in illusory super-naturalism based on irrational and dogmatic theology. The discussions by Nagel and Chalmers cogently demonstrate the inherent limitations of materialistic explanation. The materialist may willfully ignore carefully reasoned Christian critiques but they have no excuse when they fail to listen to sophisticated critique from their own camp.

    For sure we will have to include Plantinga’s evolutionary argument against naturalism, sometime in the future, but I want first to write on some substantive issues. I don’t want to be stuck with refining methodological issues ad infinitum. We can’t just keep sharpening the chopsticks; we need to try some pickings. We can sharpen further the chopsticks in due course.