Nietzsche claims that “psychology is once again the path to the fundamental problems” (BGE 23). What are these “fundamental problems”? I provide a partial answer by focusing upon the way in which psychology informs Nietzsche's account of value. I argue that Nietzsche's ethical theory is based upon the idea that power has a privileged normative status: power is the one value in terms of which all others values are to be assessed. Yet how could power have this privileged status, given that Nietzsche denies that there are any objective facts about what is valuable? Nietzsche's account of psychology provides the answer: he grounds power's privileged status in facts about the nature of human motivation. In particular, Nietzsche's account of drives entails that human beings are ineluctably committed to valuing power. So Nietzsche's ethical theory follows from his philosophical psychology.