Hume's most significant impact on eighteenth-century America was achieved, we have repeatedly been told, when in 1787 James Madison turned to Hume's political essays while working out the argument of his celebrated Federalist No. 10. The first historian to explore that ground in depth was Douglass Adair. In his long unpublished but often-cited Yale dissertation of 1943 and then in its subsequent recasting in published essays (later gathered together for a posthumous edition and since republished in various formats), Adair outlined his thesis that Hume's political essays provided the master key for unlocking the vault of Madison's political thought. Adair's analysis of this topic has been enormously influential, but it has not convinced everyone. Many Madison scholars, for instance, argue that to approach Madison's writings on faction in 1787 we need look to his experiences in Virginia, not to Hume's books. But the history of ideas is seldom as neat as Adair's story implied, nor is it as compartmentalized as his critics assume. A choice between books and experience — between theory and practice — is a false one that has seriously clouded our understanding of Hume's influence on Madison. Madison knew and absorbed Hume's writings as he experienced American factionalism in the early 1780s. Hume's History of England, in particular, influenced Madison more significantly, and in different ways, than scholars have yet come to appreciate. Hume was much more important to Madison than even Adair imagined.