Benedictus de. Spinoza’s causal axiom plays a central role in Spinoza’s arguments for some of his most significant doctrines, including that things with nothing in common cannot causally interact. Spinoza famously denies that things that do not share an attribute can causally interact. Spinoza believes that causation implies understanding because he thinks that if one thing causes another, then the one causally explains the other. The causal axiom entails that ideas about effects imply ideas about their causes. Spinoza’s denial of interaction is motivated by the thought that causation implies causal explanation and causal explanation must be framed in terms of concepts that bear inferential connections to one another. The chapter concludes by considering the underlying philosophical motivations for the causal axiom and argue that it is an expression of a coherent and attractive view of the relationship between causation and causal explanation.