This one comes from his book, Miracles; a Preliminary Study. He begins his third chapter, "The Cardinal Difficulty of Naturalism,"
... by Naturalism we mean 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 (no heel-taps) as a necessary product of the system....
One threat against strict Naturalism has recently been launched on which I myself will base no argument, but which it will be well to notice....Some modern scientists seem to think...that the individual unit of matter (it would be rash to call it any longer a 'particle') moves in an indeterminate or random fashion; moves, in fact, 'on its own' or 'of its own accord.'...Now it will be noticed that if this theory is true we have really admitted something other than Nature. It would be, indeed, too great a shock to our habits to describe them as super-natural. I think we should have to call them sub-natural. But all our confidence that Nature has no doors, and no reality outside herself for doors to open on, would have disappeared. There is apparently something outside her, the Subnatural; it is indeed from this Subnatural that all events and all 'bodies' are, as it were, fed into her. And clearly if she thus has a back door opening on the Subnatural, it is quite on the cards that she may also have a front door opening on the Supernatural -- and events might be fed into her at that door too.
I have mentioned this theory because it puts in a fairly vivid light certain conceptions which we shall have to use later on. But I am not, for my own part, assuming its truth. Those who like myself have had a philosophical rather than a scientific education find it almost impossible to believe that the scientists really mean what they seem to be saying. I cannot help thinking they mean no more than that the movements of individual units are permanently incalculable to us, not that they are in themselves random and lawless. And even if they mean the latter, a layman can hardly feel any certainty that some new scientific development may not tomorrow abolish this whole idea of a lawless Subnature. For it is the glory of science to progress. [Emphasis mine] I therefore turn willingly to other ground.
Notice that Lewis has made it clear that he will not base his apologetics on science, since science can change, but for those who accept the "truths" of science, clearly it can pose serious challenges to metaphysical positions, in this case Naturalism.
Well, if it can pose challenges to metaphysical positions, then there's no reason to think it cannot provide support for metaphysical positions. Thus we saw previously that Lewis thought scientific evidence provided support for the Christian doctrine that the universe had a beginning.
So there is no reason why Lewis would necessarily reject evidence of Intelligent Design as being unable to provide evidence that supports a Theistic worldview.
Next time we'll consider what Lewis would have thought about the "God-of-the Gaps" argument.