This is a pre-copyedited, author-produced PDF of an article accepted for publication in English Historical Review following peer review. The version of record is available online at doi: 10.1093/ehr/cev215.This article offers a more systematic assessment of John Maynard Keynes’s attitude to Liberalism, and his engagement with the Liberal Party, than has previously been attempted. It does so as a means of exploring how the study of ideologies should be approached. To Keynes, ideologies were not simply constellations of fixed principles; in his view, policies were always contingent on circumstances, and thus what was appropriate now would become outdated in the future. The Liberal, therefore, needed a flexible and experimental psychological a...