Are you a naturalist or a supernaturalist? That is, do you believe the physical cosmos is all there is and ever has been, or do you allow for some other nature, even transcendent reality, above or behind our nature? If you’re not sure which you are, or if you’re not very confident about why you are whichever you are, you could read the books and papers and articles of philosophers and thinkers on the subject going back to the beginning of early Modern naturalism and up to our contemporary time to include the broadest scope of thought on the subject. Or you could just read the opening chapters of C.S. Lewis’ Miracles.

Lewis gives a concise and lucid description of the naturalist and supernaturalist positions, defining the terms clearly and describing in rubber-meets-the-road language how they differ and why it matters. The most serious difference between the two, he notes, is their accounts of rational thought. The naturalist believes the cosmos, the solar system, the earth itself and all the life on it to be the result of non-rational, “accidental” movements of physical stuff. In this world, comets, tornadoes, and neurons in our brains are all equally set in motion because of blind, “accidental” physical events in the past. Rationality and consciousness itself must exist within this system (because nothing else exists), but that means that they too are just the result of the movements of grey matter in our brains obeying the blind laws of physics. In other words, we have no reason to believe they can help us discover truth, even if it feels like they do. Reason, consciousness, free will: all illusions, and ultimately, all absurd.

To summarize a very careful, thorough treatment, Lewis concludes (with so many others before him) that Naturalism intrinsically cannot account for rationality and reason. Supernaturalists don’t grant the terms in the naturalist idea of reality, but instead recognize that “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 [supernaturalists], 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.”

A second but closely related difference between the two philosophies is their accounts of morality and ethics. As with reason, Lewis demonstrates how if Naturalism is true, then there can be no such thing as objective right and wrong, good and evil. He explains, “If the fact that men have such ideas as ‘ought’ and ‘ought not’ at all can be fully explained by irrational and non-moral causes, then those ideas are an illusion.” He then goes on to observe a trend among flag-bearing naturalists that we see all the time in our own day, with figures such as Carl Sagan, Bill Nye, and Niel DeGrasse Tyson. It’s worth quoting at length:

The Naturalist can, if he chooses, brazen it out. He can say, ‘Yes. I quite agree that there is no such thing as wrong and right. I admit that no moral judgement can be “true” or “correct” and, consequently, that no one system of morality can be better or worse than another. All ideas of good and evil are hallucinations. . .’ Indeed many Naturalists are delighted to say this.

But then they must stick to it; and fortunately (though inconsistently) most real Naturalists do not. A moment after they have admitted that good and evil are illusions, you will find them exhorting us to work for posterity, to educate, revolutionise, liquidate, live and die for the good of the human race. A Naturalist like Mr H. G. Wells spent a long life doing so with passionate eloquence and zeal. . . . For they write with indignation like men proclaiming what is good in itself and denouncing what is evil in itself, and not at all like men recording that they personally like mild beer but some people prefer bitter. Yet if the ‘oughts’ of Mr Wells and, say, Franco are both equally the impulses which Nature has conditioned each to have and both tell us nothing about any objective right or wrong, whence is all the fervour? Do they remember while they are writing thus that when they tell us we ‘ought to make a better world’ the words ‘ought’ and ‘better’ must, on their own showing, refer to an irrationally conditioned impulse which cannot be true or false any more than a vomit or a yawn?

My idea is that sometimes they do forget. That is their glory. Holding a philosophy which excludes humanity, they yet remain human. At the sight of injustice they throw all their Naturalism to the winds and speak like men and like men of genius. They know far better than they think they know. But at other times, I suspect they are trusting in a supposed way of escape from their difficulty.

It works—or seems to work—like this. They say to themselves, ‘Ah, yes. Morality’—or ‘bourgeois morality’ or ‘conventional morality’ or ‘traditional morality’ or some such addition—‘Morality is an illusion. But we have found out what modes of behaviour will in fact preserve the human race alive. That is the behaviour we are pressing you to adopt. Pray don’t mistake us for moralists. We are under an entirely new management’…just as if this would help. It would help only if we grant, firstly, that life is better than death and, secondly, that we ought to care for the lives of our descendants as much as, or more than, for our own. And both these are moral judgements which have, like all others, been explained away by Naturalism. Of course, having been conditioned by Nature in a certain way, we do feel thus about life and about posterity. But the Naturalists have cured us of mistaking these feelings for insights into what we once called ‘real value’. Now that I know that my impulse to serve posterity is just the same kind of thing as my fondness for cheese—now that its transcendental pretensions have been exposed for a sham—do you think I shall pay much attention to it? When it happens to be strong (and it has grown considerably weaker since you explained to me its real nature) I suppose I shall obey it. When it is weak, I shall put my money into cheese. There can be no reason for trying to whip up and encourage the one impulse rather than the other. Not now that I know what they both are. The Naturalists must not destroy all my reverence for conscience on Monday and expect to find me still venerating it on Tuesday.

There is no escape along those lines. If we are to continue to make moral judgements (and whatever we say we shall in fact continue) then we must believe that the conscience of man is not a product of Nature.