Intellectual history is usually seen as essentially historical. It is – but it is also essentially philosophical, both when theorising intellectual history, which some intellectual historians do, and when interpreting texts, which all intellectual historians do. I demonstrate this symbiosis between history and philosophy via critical reflections on Martin Jay’s recent book Genesis and Validity. Philosophical analysis, closely integrated with historical examples, suggests that we should significantly rethink Jay’s theorisation of the relationship between genesis and validity (e.g. whether ideas from (...) one context are valid in others). But the symbiosis between history and philosophy matters more when interpreting texts. Philosophical analysis is a powerful tool for recovering what authors meant, understanding how their ideas fit together, and seeing similarities and differences between ideas, as I show with examples from Quentin Skinner’s interpretations of Machiavelli, Hobbes and others. Yet even Jay and Skinner – two of the world’s most philosophically astute intellectual historians – overlook the crucial symbiosis between history and philosophy. (shrink)