We reach certainty in matters of fact by direct perception, or by inferences from other facts, when they transcend the testimony of our senses and memory.  These arguments from experience are of an entirely different sort from the rational demonstrations of mathematics; as the contrary of a fact is always thinkable (the proposition that the sun will not rise to-morrow implies no logical contradiction), they yield, strictly speaking, probability only, no matter how strong our conviction of their accuracy may be.  Nevertheless it is advisable to separate this species of inferences from experience—­whose certainty is not doubted except by the philosophers—­from uncertain probabilities, as a class intermediate between the latter and demonstrative truth (demonstrations—­proofs—­probabilities).  All reasonings concerning matters of fact are based on the relation of cause and effect.  Whence, then, do we obtain the knowledge of cause and effect?  Not by a priori thought.  Pure reason is able only to analyze concepts into their elements, not to connect new predicates with them.  All its judgments are analytic, while synthetic judgments rest on experience.  Judgments concerning causation belong in this latter class, for effects are entirely distinct from causes; the effect is not contained in the cause, nor the latter in the former.  In the case of a phenomenon previously unknown we cannot tell from what causes it has proceeded, nor what its effect will be.  We argue that fire will warm us, and bread afford nourishment, because we have often perceived these causal pairs closely connected in space and time.  But even experience does not vouchsafe all that we desire.  It shows nothing more than the coexistence and succession of phenomena and events; while the judgment itself, e. g., that the motion of one body stands in causal connection with that of another, asserts more than mere contiguity in space and time, it affirms not merely that the one precedes the other, but that it produces it—­not merely that the second follows the first, but that it results from it.  The bond which connects the two events, the force that puts forth the second from the first, the necessary connection between the two is not perceived, but added to perception by thought, construed into it.[1] What, then, is the occasion and what the warrant for transforming perceived succession in time into causal succession, for substituting must for is, for interpreting the observed connection of fact into a necessary connection which always eludes observation?

[Footnote 1:  The weakness of the concept of cause had been recognized before Hume by the skeptic, J. Glanvil (1636-80).  Causality itself cannot be perceived; we infer it from the constant succession of two phenomena, without being able to show warrant for the transformation of thereafter into thereby.]