The notion of virtue is undoubtedly one of the main components of the ethics of ancient Greek thinkers. Being the privileged instrument for achieving happiness, virtue already played an important role in Archaic Greece. But it is necessary to wait for the Classical period for such a moral conception to be more democratic and accessible to the majority of citizens. The contribution of Socrates and Plato is fundamental to this change inasmuch as the conquest of happiness by means of virtue (...) can also benefit th ose who venture to use reason, especially philosophers. In this new ethical model, the reference is without hesitation Aristotle, as exemplified by his wide influence in the course of the history of philosophy. However, if the Stagirite has dominated the e thical scene for centuries, it owes much to his master Plato, who taught him notably the essentials about the nature of virtue. Even though there are important distinctions between the ethical theories of these two thinkers, particularly on the question of virtue as knowledge, the Platonic explanation of ‘what is’ virtue had fundamental repercussions on Aristotle’s own conception. In the present article, we examine Plato’s thinking on this topic of the nature of virtue in three important dialogues, namely Euthydemus, Theaetetus, and Philebus. In addition to following the likely evolution of Plato’s doctrine on this issue, the relevance of his descriptions of virtue to his most important disciple is also emphasized. (shrink)