Aquinas thinks that if we want to understand causal interactions between material substances, we cannot focus exclusively on agents and their active powers. In his view, there are also passive potencies which enable material substances to be acted upon. He claims that for every type of active potency, there is a corresponding passive potency. This article aims to clarify Aquinas’s views about the passive potencies of material substances. It recovers his thinking on three key questions: first, what is the basis (...) or source of a material substance’s passive potentialities? Put otherwise, what constituents of material substances explain why they have capabilities for being acted upon? Second, how are a material substance’s passive potencies identified and distinguished from one another? Lastly, are passive potencies for undergoing action the same as a substance’s potencies for existing in determinate ways? For example, is a pot of water’s potentiality for being heated the same as its potentiality to be hot? (shrink)