In this article, I examine the way Aristotle makes use of the methods Plato labelled as "dialectic". After suggesting a unified interpretation of Plato’s dialectic, I show that Aristotle makes room for them not inside the context of demonstrative science, but at the level of the investigation concerning the principles of such a science. These principles are, for the most part, definitions; and Plato’s dialectical methods are designed to search for and obtain definitions. Although Aristotle, contrary to Plato, seems to distinguish between dialectic and philosophy, he relates both to the same capacity, and he suggests that their methods are identical up to a certain point. Moreover, the cognitive state corresponding to dialectic is, for Aristotle as for Plato, intelligence (nous). Nevertheless, there remain important differences between Plato and Aristotle on this issue: while the dialogical dimension of dialectic is for Plato constitutive of philosophy and implies that the philosophical thought is a perpetual motion, it is according to Aristotle what distinguishes dialectic from philosophy, which must for its part come to a rest; and while philosophy presupposes a rupture with sensation according to Plato, Aristotle envisages it in continuity with sensible experience.