The chapter ‘Alexander of Aphrodisias on Concepts’ by Frans A. J. de Haas takes up another aspect of concept theory, that is, the endeavour to define what a concept is. Furthermore, he explores the interactions between the Peripatetics and the Stoics, as they are evidenced by Alexander, on ontological as well as psychological and epistemological issues. De Haas also offers a systematic study of part of Alexander’s rich vocabulary denoting concepts, thoughts, and universals, and of a correspondingly rich collection of verbs referring to the human activities of abstracting or constructing concepts. Importantly, this analysis sheds light on Alexander’s understanding of ennoia and noêma, and on Alexander’s views concerning the epistemic reliability of concepts and the unity of concepts in the human soul.