The significance of the notions of actuality and potentiality in Plotinus' thought can hardly be overstated. Throughout the Enneads, they are crucial to understanding the specific causality of intelligible realities and the relation of participation between intelligible and sensible realms. In Ennead II.5, Plotinus for the first time provides a systematic clarification of his peculiar use of these terms, through a sustained revision of Aristotle's own elaboration of the topic and of his terminology. The treatise discusses the different meanings of potentiality and actuality as well as the way each of them applies or does not apply to the sensible realm, to the intelligible realm, and to matter.