 Later empiricists could (and did) follow Ockham in denying that there are universals to be discovered; they professed to be undisturbed by the fact that we cannot grasp the “real essences” of the objects we perceive (Locke Essay 3.6.9). But the problems of empiricism did not go away. Even one of the empiricist’s favorite ideas—the “covering law” account of scientific explanation—assumes we can arrive at true generalizations, propositions that apply to all members of a particular class. But for those who take Ockham’s nominalism to its logical conclusion, a law of nature can be nothing more than a contingently true proposition. Since the similarities that fix class membership have no metaphysical grounding, not being based on a shared nature, we cannot know the truth of the law by any kind of rational insight. The only way of testing such a proposition is by observation. It is at this point that there arises the “problem of induction,” first discussed by Sextus Empiricus and picked up by Nicholas of Autrecourt.