No writer is more closely bound up with our deepest sense of the meaning of the "American" than Thomas Jefferson and it is difficult to imagine America's national purpose without some reference to his words. Yet Jefferson's projection of American identity also assumed and even constituted, of necessity, the un-American and it is in this sense that the un-American provided the necessary contours of what became the "American." Jefferson's various projects are often seen in tension with one another. But this dialectic between the American and the un-American helps reconcile many of them. Federalists, Jefferson believed, assumed that governing Americans demanded the force and corruption that had long kept Europeans in order, whereas Americans, he believed, had an experience of history that rendered them capable of transcending such political theory and practicing democratic politics. This paper explores this dialectic between the American and the un-American in Jefferson's thought as a problem of national self-definition and argues that Jefferson's overwhelming confidence about American identity rested to a large degree in the shudder produced by his experience of the other. Years before Joseph McCarthy and HUAC, Jefferson's project of defining the nation created the un-American, rendering Americans ever since profoundly, however paradoxically, ambivalent about the prospects for revolutionary republicanism abroad. [ABSTRACT FROM AUTHOR]