This paper reconstructs and analyzes Thomas Aquinas’ intriguing views on transeunt causal activity, which have been the subject of an interpretive debate spanning from the fifteenth century up until the present. In his Physics commentary, Aquinas defends the Aristotelian positions that (i) the actualization of an agent’s active potential is the motion that it causes in its patient and (ii) action and passion are the same motion. Yet, in other texts, Aquinas claims that (iii) action differs from passion and (iv) “action is in the agent” as subject. This paper proposes a solution for how to reconcile Aquinas’ varying claims about what transeunt causal activity is in reality. In addition to advancing understanding of Aquinas’ views on causal activity, the paper also offers insights into more general topics in his thought, such as the relationship between actualities and accidents and the nature of extrinsic accidents.