I examine Plato's claim in the Republic that philosophers must rule in a good city and Aristotle's attitude towards this claim in his early, and little discussed, work, the Protrepticus. I argue that in the Republic, Plato's main reason for having philosophers rule is that they alone understand the role of philosophical knowledge in a good life and how to produce characters that love such knowledge. He does not think that philosophic knowledge is necessary for getting right the vast majority of judgments about actions open to assessment as virtuous or vicious. I argue that in the Protrepticus Aristotle accepts similar reasons for the rule of philosophers, but goes beyond the Republic and seems to suggest that philosophic knowledge is required for getting right ethical and political judgments in general. I close by noting some connections with Aristotle's later views in the Eudemian Ethics, the Nicomachean Ethics, and the Politics.