The Uniformity of Nature

Every inductive inference assumes that nature is uniform — that things of the same kind behave the same way in the same circumstances. Without this assumption, no number of past observations could support a conclusion about any unobserved case. But what justifies the assumption itself? Mill acknowledges that it cannot be derived from pure reason; it is not a logical truth. Nor can it be justified by observation without circularity, since any such justification would itself presuppose what it purports to prove.

Mill accepts the apparent circularity and argues that it is tolerable. The uniformity of nature is the broadest and most confirmed of all empirical generalisations — confirmed by every successful prediction, every successful application of science to technology, every reliable inference we have ever drawn. It is not justified a priori but validated a posteriori, by the entire history of human experience. Mill concedes this is not a proof free of all presupposition; it is simply the best available response to a fundamental and irresolvable puzzle about the grounds of empirical knowledge.

The discussion of uniformity and inductive justification occupies Book III, Chapter 3 of A System of Logic. Mill's answer anticipates the position sometimes called 'pragmatic vindication' in twentieth-century philosophy of science.