There is no clear consensus among scholars about the authenticity of the Magna Moralia. Here I present a new case for thinking that the work was composed by a later Peripatetic, and is not, either directly or indirectly, the work of Aristotle. My argument rests on an analysis of the author’s usage of ἐνέργεια, which is a fruitful way to investigate the date of the work: the term was apparently coined by Aristotle but in later antiquity came to be used in ways inconsistent with Aristotle’s own usage. I argue that in several passages from the Magna Moralia the term is used in this distinctively late sense and that it is not plausible to think that this innovation could have occured in Aristotle’s own lifetime or shortly thereafter.