In Posterior Analytics II.19 Aristotle raises and answers the question, how do first principles become known? The usual view is that the question asks about the process or method by which we learn principles and that his answer is induction. I argue that the question asks about the original prior knowledge from which principles become known and that his answer is perception. Hence the aim of II.19 is not to explain how we get all the way to principles but to defend the claim that our knowledge of them originates in perception. Aristotle explains how we learn principles earlier in book II, in his account of definitional inquiry. In II.19 he explains how we reach by induction the preliminary accounts necessary for such inquiries.