There are at least five ways of treating a political theory. One is to consider it a form of intellectual exercise, pure and simple—an adventure in abstraction to sharpen the mind. Another is to go through it, and through other theories, seeking a personal practical philosophy for oneself. A third is to distil the history out of a political theory—examine what light the theory can throw on the age from which it emerged. A fourth is also to treat the theory as a source of historical data, but not in the sense in which a river may conceivably be a source of some dissolved substance from the silt it carries, but in the sense in which a river may be a source of water. In this latter sense a political theory is not distilled to yield history. It is itself part of the flow of history—part of what Americans sometimes call intellectual history.