This chapter traces the ways in which Hume’s ‘Idea of a Perfect Commonwealth’ responds not only to Harrington’s Commonwealth of Oceana (1656) but also to Montesquieu’s Spirit of the Laws (1748). The large federal constitution that Hume proposed at the end of his Political Discourses turns out to have as much in common with Montesquieu’s understanding of modern monarchy as it does with Harrington’s vision for an equal republic. Indeed, there is reason to suspect that Montesquieu’s criticism of Oceana in his chapter ‘On the English Constitution’ prompted Hume to devise his alternative version of Harrington’s commonwealth. Hume adapted Oceana’s framework for uniform electoral districts and tiers of representation to the spirit of commerce and competition that he and Montesquieu associated with modern Britain. The result was a state with ‘all the advantages of both a great and little commonwealth’.