Abstract The main thesis of the paper is that, in the coda to the Protagoras (360e-end), Plato tells us why and with what justification he demands a definition of virtue: namely, in order to resolve a particular aporia . According to Plato's assessment of the outcome of the arguments of the dialogue, the principal question, whether or not virtue can be taught , has, by the end of the dialogue, emerged as articulating an aporia , in that both protagonists, Socrates (...) and Protagoras, have argued equally on both its sides. The first part of the paper provides an extensive analysis of the coda, with the aim of establishing the main thesis. The second part provides a comprehensive review of the arguments in the dialogue, with the aim of determining whether their outcome is what Plato says in the coda that it is. I undertake this review in three steps: on Plato's conception of reasons (logoi); Socrates' arguing on both sides; and Protagoras' arguing on both sides. (shrink)