The title of this essay poses not one vexing issue but two, and each of them sharply challenges the student of American political thought. The first might be called the common problem of political theory—the question of its relevance to the institutional facts of life. How, it is asked, can the analysis of political ideas help to illuminate our understanding of political action? Can theory lead us to a surer knowledge of why governments and electorates behave as they do? Can it help us to diagnose and prescribe? Or is the study of theory, on the contrary, justified simply on the ground that the words of Plato and Hobbes and Locke are part of what Matthew Arnold called culture: “the best that has been thought or known in the world”? This is, I take it, a problem universal among students of political thought, whether they choose America, Europe, or China as their realm; and it lends itself to no easy answers.