"I have known only one person in my life who claimed to have seen a ghost. It was a woman; and the interesting thing is that she disbelieved in the immortality of the soul before seeing the ghost and still disbelieves after having seen it. She thinks it was a hallucination. In other words, seeing is not believing. This is the first thing to get clear in talking about miracles. Whatever experiences we may have, we shall not regard them as miraculous if we already hold a philosophy which excludes the supernatural. Any event which is claimed as a miracle is, in the last resort, an experience received from the senses; and the senses are not infallible. We can always say we have been the victims of an illusion; if we disbelieve in the supernatural this is what we always shall say. Hence, whether miracles have really ceased or not, they would certainly appear to cease in Western Europe as materialism became the popular creed. For let us make no mistake. If the end of the world appeared in all the literal trappings of the Apocalypse, if the modern materialist saw with his own eyes the heavens rolled up and the great white throne appearing, if he had the sensation of being himself hurled into the Lake of Fire, he would continue forever, in that lake itself, to regard his experience as an illusion and to find the explanation of it in psycho-analysis, or cerebral pathology. Experience by itself proves nothing. If a man doubts whether he is dreaming or waking, no experiment can solve his doubt, since every experiment may itself be part of the dream. Experience proves this, or that, or nothing, according to the preconceptions we bring to it.
This fact, that the interpretation of experiences depends on preconceptions, is often used as an argument against miracles. It is said that our ancestors, taking the supernatural for granted and greedy of wonders, read the miraculous into events that were really not miracles. And in a sense I grant it. That is to say, I think that just as our preconceptions would prevent us from apprehending miracles if they really occurred, so their preconceptions would lead them to imagine miracles even if they did not occur. In the same way, the doting man will think his wife faithful when she is not and the suspicious man will not think her faithful when she is: the question of her actual fidelity remains, meanwhile, to be settled, if at all, on other grotmds. But there is one thing often said about our ancestors which we must not say. We must not say ‘They believed in miracles because they did not know the Laws of Nature.’ This is nonsense. When St Joseph discovered that his bride was pregnant, he was ‘minded to put her away’. He knew enough biology for that.
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The experience of a miracle in fact requires two conditions. First we must believe in a normal stability of nature, which means we must recognize that the data offered by our senses recur in regular patterns. Secondly, we must believe in some reality beyond Nature. When both beliefs are held, and not till then, we can approach with an open mind the various reports which claim that this super- or extra-natural reality has sometimes invaded and disturbed the sensuous content of space and time which makes our ‘natural’ world. The belief in such a supernatural reality itself can neither be proved nor disproved by experience. The arguments for its exist ence are metaphysical, and to me conclusive. They turn on the fact that even to think and act in the natural world we have to assume something beyond it and even assume that we partly belong to that something. In order to think we must claim for our own reasoning a validity which is not credible if our own thought is merely a function of our brain, and our brains a by-product of irrational physical processes. In order to act, above the level of mere impulse, we must claim a similar validity for our judgments of good and evil. In both cases we get the same disquieting result. The concept of nature itself is one we have reached only tacitly by claiming a sort of super-natural status for ourselves.If we frankly accept this position and then turn to the evidence, we find, of course, that accounts of the supernatural meet us on every side. History is full of them---of ten in the same documents which we accept wherever they do not report miracles. Respectable missionaries report them not infrequently. The whole Church of Rome claims their continued occurrence. Intimate conversation elicits from almost every acquaintance at least one episode in his life which is what he would call ‘queer’ or ‘rum’. No doubt most stories of miracles are unreliable; but then, as anyone can see by reading the papers, so are most stories of all events. Each story must be taken on its merits: what one must not do is to rule out the supernatural as the one impossible explanation."
(The Timeless Writings of C.S. Lewis: God in the Dock :312-313)