He draws the line between certainty and probability at the same point as Locke; but is more definite as to probability, 226. In thus leaving mathematics and a limited sort of experimental physics (limited by the exclusion of all general inference from the experiment) out of the reach of his scepticism, and in making them his basis of attack upon what he conceived to be the more pretentious claims of knowledge, Hume was again following the course marked out for him by Locke. It will be remembered that Locke, even when his ‘suspicion’ of knowledge is at its strongest, still finds solid ground _(a)_ in ‘particular experiments’ upon nature, expressed in singular propositions as opposed to assertions of universal or necessary connexion, and _(b)_ in mathematical truths which are at once general, certain, and instructive, because ‘barely ideal.’ All speculative propositions that do not fall under one or other of these heads are either ‘trifling’ or merely ‘probable.’ Hume draws the line between certainty and probability at the same point, nor in regard to the ground of certainty as to ‘matter of fact or existence’ is there any essential difference between him and his master. As this ground is the ‘actual present sensation’ with the one, so it is the ‘impression’ with the other; and it is only when the proposition becomes universal or asserts a necessary connection, that the certainty, thus given, is by either supposed to fail. It is true that with Locke this authority of the sensation is a derived authority, depending on its reference to a ‘body now operating upon us,’ while with Hume, so far as he is faithful to his profession of discarding such reference, it is original. But with each alike the fundamental notion is that a feeling must be ‘true _while it lasts_,’ and that in regard to real existence or matter of fact no other truth can be known but this. Neither perceives that a truth thus restricted is no truth at all--nothing that can be stated even in a singular proposition; that the ‘particularity in time,’ on which is supposed to depend the real certainty of the simple feeling, is just that which deprives it of significance [1]--because neither is really faithful to the restriction. Each allows himself to substitute for the momentary feeling an object qualified by relations, which are the exact opposite of momentary feelings. ‘If I myself see a man walk on the ice,’ says Locke (IV, xv. 5), ‘it is past probability, it is knowledge:’ nor would Hume, though ready enough on occasion to point out that what is seen must be a colour, have any scruple in assuming that such a complex judgment as the above so-called ‘sight’ has the certainty of a simple impression. It is only in bringing to bear upon the characteristic admission of Locke’s Fourth Book, that no general knowledge of nature can be more than probable, a more definite notion of what probability is, and in exhibiting the latent inconsistency