The article presents the first comprehensive interpretation of the ergon argument in Aristotle’s Protrepticus. It further argues that Aristotle in this argument distinguishes the ergon of an entity from the ergon of its virtue thus presenting a complicated argumentative structure which is explicitly simplified in the Eudemian Ethics. Based on the latest attempts to reconstruct the Protrepticus, the article shows the relation of the ergon argument to its other versions in both Ethics. This account not only clarifies the relation of (...) the Protrepticus to Aristotle’s other ethical writings, it shows that Aristotle intends the argument as a transition between describing what human being does and what a good human being does well, namely a natural transition from the descriptive to the normative. (shrink)