Metaphysical Animals: How Four Women Brought Philosophy Back to Life

by Clare Mac Cumhaill

‘With this argument you propose to destroy “naturalism”.’[86] In that chapter, Lewis had argued that ‘naturalists’ cannot account for thought’s rational character. Thought, he had argued, is governed by rational relations. Conclusions follow logically from premises. Yet the naturalist insists that all relations are ultimately causal. Human behaviour and thought follow causally, not rationally, from other physical events. Lewis claimed that this position is self-defeating. The naturalist thinks he has rational grounds for his belief in naturalism. But if naturalism is true, then his belief has no rational grounds at all. It is just a physical event, with a causal explanation like any other. A belief without rational grounds is irrational, so the naturalist’s position, Lewis concludes, is self-undermining.