«Concept» is a central notion in modern philosophy that also influences other disciplines like psychology and linguistics. The author compares modern theories to the work of Aristotle as the first philosopher with an extensive corpus and one of the predecessors both of classical theory and of modal theories of «concepts». It is surprising that there is no equivalent term for «concept» in his work. Both pathêma and logos are central to his theory of language and thought. Therefore, this book describes which notion in Aristotle's writing comes closest to «concept» and whether or not it generates a precise theory.