C.S.Lewis & Anscombe

Philosophers and scientists including Victor Reppert, William Hasker, and Alvin Plantinga have expanded on the “Argument from Reason” and credit Lewis with first bringing the argument to light in Miracles.
The argument holds that if, as thoroughgoing naturalism entails, all of our thoughts are the effect of a physical cause, then there is no reason for assuming that they are also the consequent of a reasonable ground. Knowledge, however, is apprehended by reasoning from ground to consequent. Therefore, if naturalism were true, there would be no way of knowing it, or anything else not the direct result of a physical cause.
Lewis asserts that by this logic, the statement “I have reason to believe naturalism is valid” is self-referentially incoherent in the same manner as the sentence “One of the words of this sentence does not have the meaning that it appears to have”, or the statement “I never tell the truth”. In each case, to assume the veracity of the conclusion would eliminate the possibility of valid grounds from which to reach it. To summarize the argument in the book, Lewis quotes J. B. S. Haldane who appeals to a similar line of reasoning. Haldane states “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.”
The original version of Miracles contained a different version of chapter 3 entitled “The Self-Contradiction of the Naturalist.” In it, Lewis made the same argument but referred to atomic motions in the brain as “irrational.” In a Socratic Club debate, G.E.M. Anscombe criticized this, prompting Lewis to revise the chapter. The revised chapter presents a more detailed elucidation of the argument and distinguishes between “non-rational” and “irrational” processes. G.E.M. Anscombe commented on the process after Lewis’s death that the rewrite showed “honesty and seriousness” on the part of Lewis.(Wikipedia)

The Anscombe myth

In addition to explaining and expanding Lewis’s theistic argument for God, Dr. Reppert has also made an important contribution to Lewis studies by deconstructing what he calls the “Anscombe myth”. Roughly, the “Anscombe myth” arose, in part, from an actual encounter C. S. Lewis had at his Socratic Club with Catholic philosopher G. E. M. Anscombe, over the soundness of the theistic argument he presents in his book Miracles. It has been alleged that Elizabeth Anscombe, in her presentation of the perceived problematic areas in Lewis’s argument, had so thoroughly discredited his argument that Lewis sank into apologetic and theological obscurity. It has also been suggested that this friendly encounter led Lewis to not only reject the Argument from Reason, but also significantly question the validity of Christianity altogether. Dr. Reppert, in his critique of the “Anscombe myth”, points out that Lewis merely revised his argument for later editions of Miracles, rather than reject it. Furthermore, Reppert notes that Lewis continued to proactively maintain the argument, as evidenced by the publication of several post-Anscombe-debate articles; chiefly in Christian Reflections and God in the Dock. Reppert also points out that Lewis’s spiritual tenor in his later writings doesn’t significantly differ in tone or substance from his earlier Christian material.
(Wikipedia)

The Argument from Reason (1998)
Victor Reppert

The simplest way to generate this kind of argument against materialism is to say that since all thoughts are determined (apart from pure quantum chance) by the nonpurposive motion of atoms in the brain, it follows that such a process cannot be a rational inference. But such an argument would be, of course, rather too simple. As Anscombe pointed out in response to C. S. Lewis’s early version of the argument:

“Your idea appears to be that ‘the explanation’ is everywhere the same one definite requirement, so that we can know, when it is filled, that, if it has been correctly filled, the whole subject of ‘explaining this fact’ has been closed.“

But of course, many explanations for the same event can be compatible with one another. If I want to know why the soda-can is on the bookshelf, I can say “because I put it there yesterday,’ or ” because I wanted it recycled” or “because of the law of gravity” or “because it is cylindrical and not spherical.” But in the case of an instance of rational inference, we want to know whether an event can be at the same time, the motion of brain matter in a mechanistic universe, and, at the same time, the inference to a conclusion from its premises.
We know from science that the same process can be given distinct explanations depending on the level at which we are analyzing it. Physics looks at subatomic particles, while biology looks at something as an entire system. If I were to punch someone in the mouth, one could give an explanation for this punching in terms of subatomic particles, biochemistry, or gross anatomy. And it might be that the punch could also be explained in psychological terms, and sociologist could come along and tell me that members of such and such a social group are more likely to engage in this kind of aggressive behavior under such and such circumstances. These levels of explanation involve purpose, however, and the compatibility of mechanism and purpose is just what is at issue here. But what is clear enough is that different explanations, at different levels, can be given for one and the same causal transaction.
So most rebuttals to the argument from reason make the claim that explanation of a mental act of rational inference in terms of particle physics that makes no reference to reasons, and the explanation of that same act in terms of propositional attitudes, intentionality, truth, and laws of logic, are compatible with one another. Given a proper relationship between the explanation given at the physical level, the explanation at the mental level can obtain also, and one needs no more to choose between a mental and a physical explanation than one needs to choose between a biological and a physical explanation

(http://www.infidels.org/library/modern/victor_reppert/reason.html)