Hume follows Newton in replacing the mechanical philosophy's demonstrative ideal of science by the Principia's ideal of inductive proof (especially as formulated in Newton's Rule III); in this respect, Hume differs sharply from Locke. Hume is also guided by Newton's own criticisms of the mechanical philosophers' hypotheses. The first stage of Hume's skeptical argument concerning causation targets central tenets of the mechanical philosophers' (in particular, Locke's) conception of causation, all of which rely on the a priori postulation of a hidden configuration of primary qualities. The skeptical argument concerning the causal inductive inference (with its implicit principle that nature is, in Newton's words, "ever consonant with itself") then raises doubts about what Hume himself regards as our very best inductive method. Hume's own "Rules" (T 1.3.15) further substantiate his reliance on Newton. Finally, Locke's distinction between "Knowledge" and "Probability" ("Opinion") does not leave room for Hume's Newtonian notion of inductive proof.