 As no number of examined instances logically entails the conformity of unexamined instances, a universal law's unrestricted generalization bears no formally logical basis, but one justifies it by adding the principle uniformity of nature—itself unverified—thus a major induction to justify a minor induction.This apparent obstacle to empirical science was later termed the "problem of induction".For Hume, humans experience sequences of events, not cause and effect, by pieces of sensory data whereby similar experiences might exhibit merely constant conjunction—"first an event like A, and always an event like B"—but there is no revelation of causality to reveal either necessity or impossibility.