This chapter explores a hiatus in the classical use and interpretation of the notions of essence (substance), potency, and act in the reading traditions of Aristotle’s treatises on physics. Indeed, each of these terms are used in multiple ways. It is therefore important to contextualise the places where they are used as precisely as possible to make the necessary semantic distinctions. While the term act designates the realised state of an operator in full possession of his means of action, it also designates this same operator in activity and, in certain cases, the operation, activity, or force in action itself. In the case of Philoponus’ treatment of light in his lengthy commentary on Aristotle’s definition of light as the act (ἐνέργεια) of the diaphanous as diaphanous, it is this third meaning that is at issue. I will examine this usage here, focusing on Philoponus’ interpretation of the third meaning of the act, which I will call energeia, for reasons I will explain below; in addition, I will trace the reception of Philoponus’ Commentary in Michael Psellus, a prolific Byzantine thinker of the eleventh century. Philoponus is by no means the inventor of this meaning of energeia, but he may be the first to have attempted to explain this meaning of light in such a methodical way, by subjecting it to applied mathematics such as optics and catoptrics, which, regrettably, are still presented too briefly in his work.