Anscombe and Lewis

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

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