The present article seeks to demonstrate that the passage in Aristotle’ Metaphysics Theta (1048b18-35) which defends a distinction between motion (kinêsis) and activity (energeia) was not only well known by ancient commentators but also generated intense debate. The debate that is the focus here is the one caused by Plotinus’s critique of the kinêsis/energeia distinction, which can be shown to have the passage in Metaphysics Theta as its target, and the response of Iamblichus as recorded and seconded by Simplicius. This debate helps us understand the significance of the passage and clarifies its ontological stakes. That the passage dropped out of one branch of manuscripts and was therefore unknown to later commentators like Michael of Ephesus and Thomas Aquinas is arguably unfortunate; an appendix seeks to show through the examples of these two figures just what was lost.