The nature of critical thinking remains controversial. Some recent accounts have lost sight of its roots in the history of philosophy. This article discusses critical thinking in its historical and social contexts, and in particular, for its educational and political significance. The writings of Plato and Aristotle are still vital in considering what makes certain kinds of thinking and certain kinds of knowledge distinctive. But neither Plato nor Aristotle theorised critical thinking in its specificity, that is, by differentiating it from other kinds of thinking and by outlining its features. It was only with German Idealism, as part of the European Enlightenment, that philosophy achieved a level of self-reflection and engagement that broadened, deepened and made radical the logos of Socrates and Plato. In the encounter between Modern German and Ancient Greek philosophy we can see that there are different kinds of reason, the most politically and existentially significant of which runs through critical thinking, which we can only ever experience as critical dialogue. The logos of dialectical reason is uncompromising in its pursuit of truth and "the good life," and as such, demands courage from those who pursue it.