How many ἀρεταί in Plato's Protagoras?

A large debate around Plato’s Protagoras concerns the so-called ‘unity of virtue’, namely Socrates’ ultimate position on the relation between ἀρετή and the five concepts (justice, piety, prudence, wisdom, and courage) connected with it. In this article, I will draw on the language and ideas of the dialogue to question the semantic presupposition which all the scholars involved in the debate, whatever their views, have been sharing, namely that the five concepts can be considered as ἀρεταί, and not just as ‘good qualities’ (τὰ καλά), or ‘parts’ (μόρια), or ‘names’ (ὀνόματα) of ἀρετή, as Plato terms them.