The argument from reason

Elizabeth Anscombe, then an up-and-coming Catholic philosopher, had a celebrated disagreement with C. S. Lewis at the Socratic Club. As a result, Lewis reformulated one of his arguments in Miracles. Years later, Anscombe offered her assessment of Lewis's reformulation:

There is one thing I will say, which is that there is a quotation by J. B. S. Haldane...The quotation runs like this: "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."

Now I want to consider the quotation all by itself. Let us suppose that it makes sense to say that mental processes–and this means everything they are inclined to call mental processes–are determined, determined wholly, by the motions of atoms in one's brain. That is, let us forget about the difficulties that might be raised about this. I mean the difficulties of taking about mental processes and when they are supposed to be determined.

In order to keep any such difficulties out of view, let us consider an analogous supposition, namely that it makes sense to say that linguistic marks–that is, marks that are parts of a language as they occur in a printed book–are wholly determined by the machinery that printed the book. Well, it might be a book. I don't think this book [in my hand] has got any pencil notes in it or anything that might be linguistic matter that isn't printed.

This analogue has the advantage of certainly making sense, that is, that the linguistic marks occurring in this book are wholly determined by the machinery that printed the book. And indeed, it's got the advantage of not just certainly making sense, but of being true. Only we wouldn't dream of saying: if that is true, we have no reason to suppose that any of the things said in the book are true or are false, or anything like that.

Well, this illustrates the way in which a thought–a thought that somebody puts forward–trades on a mysteriousness about its objects. In the case of Haldane's remark, the mysterious objects are "mental processes"; "If every bit of every mental process is determined by motions of atoms, then I have no reason to suppose that my beliefs are true". If we change the example to something lacking the mysteriousness of "mental processes", for example to the existence of the print in a printed book, as I have in my analogue, then we observe two things. First, that the supposition that this is wholly determined by the machinery, the printing machinery, is true. And second, that that has no bearing whatever on whether anything said in the book is true, or whether we have reason or no reason to think so. Elizabeth Anscombe, "C. S. "Lewis's Rewrite of Chapter III of Miracles." Roger White, Judith Wolfe, & Brendan Wolfe, C. S. Lewis and His Circle: Essays and Memoirs from the Oxford C.S. Lewis Society (Oxford University Press 2015), 15-16.

I think both she and Lewis are right in different respects:

i) She's right to point out that determinism as such doesn't undermine reason. It depends on what lies behind the determinate outcome. And her example nicely illustrates why it would be fallacious to infer that determinism per se undermines reason. 

ii) However, I don't know that Lewis intended to show that determinism in general undercuts reason. In context, he's targeting atheism. He uses Haldane's statement as a frame of reference. Haldane was an atheist. Lewis's contention, as I see it, is that determinism in combination with naturalism undercuts reason. In particular, that blind physical determinism undercuts reason.

I think his intuition is sound, and subsequent philosophers like Victor Reppert (i.e. the argument from reason) and Alvin Plantinga (i.e. the evolutionary argument against naturalism) have developed more sophisticated versions of his rudimentary argument.