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Reason as a Universal Constant

Stuart Greenstreet asks if C.S. Lewis was right that reason proves the supernatural.

C.S. Lewis (1898-1963) was one of the most influential writers of his day – an ‘intellectual giant’, it was said. He had, and still has, a vast audience for his children’s fiction (The Chronicles of Narnia) and for his many books written to counter objections to religious belief (notably Mere Christianity). Lewis taught literature at Oxford and Cambridge Universities all his adult life, and was made a Cambridge professor in 1954. That he was also a deep and lucid philosopher is evident from his book Miracles (1947). Here he built maybe the first logically sound and convincing argument for the existence of something in addition to nature, “which we may call the supernatural.” His argument is analysed below. Will it convince you? If it is convincing, then it has serious implications for those like Richard Dawkins who vehemently deny anything in addition to nature.

Reasoning Beyond Nature

Can all natural phenomena ultimately be explained by science – even the physical necessity we observe to govern the behaviour of all natural things everywhere? Will science one day find out why gravity and the speed of light and the other fundamental physical constants are constant, and are also fine-tuned for intelligent life? No one yet knows how the constants came into being or why they are as they are, and so nature’s laws seem to lack an accessible basis. Yet if the values of those constants had been different, neither our world nor life as we know it could have come into being. So the fundamental constants are the ‘givens’ that set the very framework of nature within which all events appear to have only natural causes, and wherein science is done. This is the arena in which naturalism prevails.

C.S. Lewis defined naturalism as “the doctrine that only Nature – the whole interlocked system – exists. And if that were true, every thing and event would, if we knew enough, be explicable without remainder … as a necessary product of the system” (p.18). Lewis wrote these words in his book Miracles (1947). Here he grants that there can be no miracles unless there exists something else in addition to nature “which we may call the supernatural.” This distinction, he explains, is not between mind and matter, much less between soul and body, but between nature and “something else” – something which Lewis believes has to exist in addition to nature, and which he aims to identify.

To Lewis, a miracle would be “an interference with Nature by supernatural power” (p.5). Lewis’s definition is crucially different to the one David Hume used in his celebrated essay Of Miracles (1777) – namely, that a miracle would be a “violation of the laws of nature.” This is still the popular idea of a miracle: that it is a happening in which the laws of physics or biology are suspended.

Lewis explicitly denies this. “We are in the habit of talking as if the laws of Nature caused events to happen; but they have never caused any event at all … They state the pattern to which every event – if only it can be induced to happen – must conform” he writes in Miracles on p.93. So a miracle would not violate or suspend nature’s laws, but would rather feed new events into nature. A miracle would occur if a supernatural cause was somehow fed into nature and digested – just like any other cause – by nature’s law-like system.

For Lewis naturalism would entail determinism. His view of nature is of a regime in which everything that happens depends on something else happening within the system, and ultimately on the whole system of interlocking events. To show that miracles are possible, then, Lewis needs to prove that something exists which neither depends on nature’s interlocking system, nor could be explained as being a necessary product of it. This singular exceptional item, he decides, is rational thought, “which is not part of the system of Nature”:

“Acts of reasoning are not interlocked with the total interlocking system of Nature as all its other items are interlocked with one another. They are connected with it in a different way; as the understanding of a machine is certainly connected with the machine, but not in the way the parts of the machine are connected with each other. The knowledge of the thing is not one of the thing’s parts. In this sense something beyond Nature operates whenever we reason.” (pp.37-38; my italics)

And so he decides that the distinction between the supernatural and the natural is actually between Reason and Nature, “the frontier coming not where the ‘outer world’ ends and what I would ordinarily call ‘myself’ begins, but between reason and the whole mass of non-rational events, whether physical or psychological.” (p.38)

To justify this conclusion, Lewis needs to prove that if all events, including crucially mental events (acts of thinking), were in fact causally determined, then we could never decide anything by logical reasoning. We could never do so, he says, because rational judgements do not depend on a causal relation between causes and their effects, but on a logical relation between premises and the conclusions we infer from them. Lewis will then need a further argument to prove that logical reasoning is not itself a natural capacity in the same way that eyesight and hearing are definitely natural, since if reasoning was natural in the same way, it would be subject to natural causes in the way our senses are. He believes our power of reasoning did not come about in the same way as our five senses: it was not evolved in us by a process of natural selection. But why should anyone believe that the power of reason is not simply a product of natural selection?

Supernatural Reasoning

Lewis begins his argument by claiming that all possible knowledge of what is true depends on the validity of reasoning: “Unless human reasoning is valid no science can be true” he says in Miracles on p.21.

Now a train of reasoning is valid, that is, has value as a means of finding truth, only if each step is connected with what went before in a ground-consequent relation. The easiest way of illustrating this relation, Lewis suggests, is to notice two distinct senses of the word because. We can say, “Grandfather is ill today because he ate lobster yesterday.” We can also say, “Grandfather must be ill today because he hasn’t got up yet (and we know he is an invariably early riser when he is well).” In the first sentence because indicates the causal relation of cause and effect: the eating made him ill. In the second, it indicates the logical relation of ground and consequent: the old man’s late rising is the reason why we believe him to be unwell. One indicates a connection between events or state of affairs, the other a logical relation between beliefs or assertions. Unless a conclusion is the logical consequent from a ground, it will be worthless and could be true only by a fluke. Thus conclusions depend on logic rather than on physical causes for their validity, even if those physical causes are, for example, previous states of the brain.

Although Lewis never refers to it, Immanuel Kant had advanced precisely this argument 160 years earlier in his Groundwork for the Metaphysics of Morals (1785). There Kant wrote, “We cannot possibly conceive of a reason as being consciously directed from outside in regard to its judgements. If a rational being were conscious of any such external influence, he would regard his judgements as determined, not by reason, but by impulse. Reason must – if it is to be reason at all – regard itself as the author of its own principles independently of external influences.” (p.448) If every judgement which is the conclusion of an argument was caused (i.e., determined) solely by previous mental/brain events and yet was not a rational insight into a connection between premises and conclusion, there would be no difference between valid and invalid inferences, and ultimately there could be no truth. In that case a doctrine of naturalism which entailed causal determinism could not be accepted as true, nor could any argument in its defence be accepted as valid. Hence Lewis’s own claim that “Unless human reasoning is valid no science can be true.” But he took it as self-evident (as presumably we all do) that human beings are able to make valid rational inferences and do form true beliefs.

Lewis’s argument reduces to this:

1) Naturalism (defined as the doctrine that only nature exists) entails determinism.

2) If naturalism is true our beliefs are held on the basis of non-rational (i.e. deterministic) causes, and we would not be able to make inferences.

3) In that case we are not able to cite reasons to justify holding our beliefs.

4) But it is incontestable that we do in fact reach truths by logical inferences.

5) Therefore we must either reject naturalism as false, or stop taking for granted that we reach true beliefs by logical inferences.

6) We cannot stop taking for granted that our beliefs are generally true.

7) Therefore we must conclude that naturalism is false, and that something else exists in addition to nature.

Unsound Evolutions

Lewis thought that this refuted naturalism and proved the truth of supernaturalism. However, as Kant knew, although this argument is logically valid, it nevertheless may be unsound. The second premise could be false. Even if naturalism were true, and all our thoughts and beliefs were causally determined by antecedent events, we might still be able to make inferences. Rational thinking was surely conducive to survival and reproduction in our ancestors, hence a practice which natural selection is bound to preserve and refine. If there is nothing but nature, one would expect reason to have come into existence by a historical process. So Lewis saw that he had to disprove the claim that “The type of mental behaviour we now call rational thinking or inference must have been ‘evolved’ by natural selection, by the gradual weeding out of types less fitted to survive.” (p.28).

Natural selection operates by eliminating biologically harmful responses and preserving responses which tend to aid survival. But how can any biological improvement in responses ever turn them into acts of logical insight – into a power of seeing how a valid argument’s conclusion must follow from its premises? The relation between response and stimulus is categorically different from that between knowledge and the truth known: “Our physical vision is a far more useful response to light than that of the cruder organisms which have only a photo-sensitive spot. But neither this improvement nor any possible improvements we can suppose could bring it an inch nearer to being knowledge of light. It is admittedly something without which we could not have had that knowledge. But the knowledge is achieved by experiments and inferences from them, not by refinement of the response. It is not men with specially good eyes who know about light, but men who have studied the relevant sciences.” (Miracles, p.29.)

Vision is a physical or bodily response, but our psychological responses to our environment – our curiosities, aversions, delights, expectations – might likewise be indefinitely improved without ever becoming anything other than responses. If our psychological responses (in contrast to our logical insight) were slowly perfected by natural selection, then that might count as a different method for achieving survival – as an ‘alternative to reason’: “A conditioning which secured that we never felt delight except in the useful or aversion save from the dangerous, and that degrees of both were exquisitely proportional to the degree of real utility or danger in the object, might serve us as well as reason or in some circumstances better,” Lewis writes on p.29. But even if such refinement of our non-rational psychological responses did happen, it could never convert them from being mere reactions to a cause into being valid inferences.

Finally Lewis considers the possibility that although reason did not evolve through natural selection, it may have been produced naturalistically through experience – originally individual experience, but the results passed on by tradition and instruction. For instance, if we often experienced finding fire (or the remains of a fire) where we had seen smoke, this would condition us to expect fire whenever we saw smoke. This expectation, expressed as ‘If smoke, then fire’ has become what we call an inference. “It might be held that this [conjunction of experiences], in the course of millennia, could conjure the mental behaviour we call reason – in other words, the practice of inference – out of mental behaviour which was not originally rational” Lewis writes on p.29. Thus experience produces expectations: it will induce us to expect fire when we see smoke just as it once induced us to expect that all swans would be white (until we saw a black one), or that water would always boil at 100ºC (until we tried a picnic on a mountain). However, such expectations were not valid inferences for they turned out to be false:

“The assumption that things which have been conjoined in the past will always be conjoined in the future is the guiding principle not of rational but of animal behaviour. Reason comes in precisely when you make the inference ‘Since always conjoined, therefore probably connected’ and go on to attempt the discovery of the connection. When you have discovered what smoke is, you may then be able to replace the mere expectation by a genuine inference. Till this is done, reason recognises the expectation as a mere expectation.”
(Miracles, p.30)


We granted earlier that Lewis’s primary argument is logically valid, but doubted the truth of its second premise. Isn’t it possible, we asked, even if naturalism is true, that an ability to think rationally could be the product of natural selection, or even of experience? Lewis’s answer is firmly negative. Evolution and/or experience equipped us to foresee causal connections between events, but not to see how things outside our own minds logically ‘must’ be. The power of reason is therefore not part of the system of nature.

Did Lewis succeed in producing possibly the first ever logically sound proof of the supernatural – “something beyond Nature” which operates whenever we reason? Almost by definition, a sound argument is one that persuades or convinces you to believe that its conclusion is true. Are you persuaded? It comes down to an essentially personal judgement.

If, as I believe, Lewis is right that human reason wasn’t made by either natural selection or experience, then is it a ‘given’, just as the fundamental physical constants are givens? Both the constants and reason seem to be distinct from nature. Like the constants, reason is a prerequisite of science: it is its most basic tool – for without rational inference there could be no truth, and so no science could be true. And reason is not only as necessary as the physical constants; it is also – again like them – universal and constant. It is certainly true that without the combination of the physical constants and human reason, life as we now know it on this planet could not have come into being.

© Stuart Greenstreet 2012

Stuart Greenstreet, a business manager and writer by trade, began philosophy in the evenings at Birkbeck College, London, before graduating from the Open University, followed by further philosophy at the University of Sussex.

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