Simplicius’ Commentary on Aristotle’s Categories contains the most extended testimony about the Stoic conception of acting (ποιεῖν) and undergoing (πάσχειν). Simplicius ascribed to the Stoics the idea that acting and undergoing are to be reduced to the movement. To this opinion Simplicius opposed the Aristotelian view according to which acting and undergoing are two different categories. In this paper I intend to outline the original Stoic position comparing the reportage of Simplicius with other Stoic sources. Later, I will deal with (...) Boethus’ defense of the distinction between the categories of acting and undergoing. I will argue that Boethus directly reacted against the Stoic opinion reformulating it in Aristotelian language. (shrink)