That there was a continuity between medieval political thought and the body of systematic theory that surrounded the Declaration of Independence and the Constitution is by now a commonplace. But when we speak of the medieval contribution to the American political tradition, it is important to avoid the implication that what medieval thought contributed was identical with what American thought received. Between the close of the fifteenth century and the latter part of the eighteenth lie some two and a half centuries of crowded thought and experience, which more or less profoundly changed the meaning of concepts continuously in use. The more we learn of medieval theory, the clearer it becomes that it must be interpreted in its own terms rather than in terms of its derivatives. And the American political tradition, of course, cannot be fully understood in terms of its historic roots. Perhaps the chief service which the history of ideas can offer to political theory lies in providing material for the sharpening of concepts through a comparative analysis. For the full understanding of the meaning of an idea, one needs to know not only what it is, but also, I suggest, what it is not. Thus there may be value in an attempt to define the medieval meaning of some concepts that were a significant part of the medieval contribution: in particular, sovereignty, natural law and natural rights, and consent.