Argument from reason

The argument from reason is an argument against naturalism and for the existence of God (or at least a supernatural being that is the source of human reason). The best-known defender of the argument is C. S. Lewis. Lewis first defended the argument at length in his 1947 book, Miracles: A Preliminary Study. In the second edition of Miracles (1960), Lewis substantially revised and expanded the argument.

Metaphysical (or philosophical) naturalism is the view that nature as studied by the natural sciences is all that exists. Naturalists deny the existence of a supernatural God, souls, an afterlife, or anything supernatural. Nothing exists outside or beyond the physical universe.

The argument from reason seeks to show that naturalism is self-refuting, or otherwise false and indefensible.

According to Lewis,

One absolutely central inconsistency ruins [the naturalistic worldview].... The whole picture professes to depend on inferences from observed facts. Unless inference is valid, the whole picture disappears.... [U]nless Reason is an absolute--all is in ruins. Yet those who ask me to believe this world picture also ask me to believe that Reason is simply the unforeseen and unintended by-product of mindless matter at one stage of its endless and aimless becoming. Here is flat contradiction. They ask me at the same moment to accept a conclusion and to discredit the only testimony on which that conclusion can be based.

More precisely, Lewis's argument from reason can be stated as follows:

1. No belief is rationally inferred if it can be fully explained in terms of nonrational causes.

Support: Reasoning requires insight into logical relations. A process of reasoning (P therefore Q) is rational only if the reasoner sees that Q follows from, or is supported by, P, and accepts Q on that basis. Thus, reasoning is trustworthy (or "valid", as Lewis sometimes says) only if it involves a special kind of causality, namely, rational insight into logical implication or evidential support. If a bit of reasoning can be fully explained by nonrational causes, such as fibers firing in the brain or a bump on the head, then the reasoning is not reliable, and cannot yield knowledge. Consider this example: Person A refuses to go near the neighbor’s dog because he had a bad childhood experience with dogs. Person B refuses to go near the neighbor’s dog because one month ago he saw it attack someone. Both have given a reason for staying away from the dog, but person A’s reason is the result of nonrational causes, while person B has given an explanation for his behavior following from rational inference (animals exhibit patterns of behavior; these patterns are likely to be repeated; this dog has exhibited aggression towards someone who approached it; there is a good chance that the dog may exhibit the same behavior towards me if I approach it). Consider a second example: person A says that he is afraid to climb to the 8th story of a bank building because he and humans in general have a natural fear of heights resulting from the processes of evolution and natural selection. He has given an explanation of his fear, but since his fear results from nonrational causes (natural selection), his argument does not follow from logical inference.

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