Abstract

Descartes believed that causation is intelligible only if the cause and effect are similar, since it is impossible to understand how the reality of an effect can owe anything to the reality of its cause if the two have nothing in common. I argue first that Descartes has a coherent and reasonably strong metaphysical justification for his condition of causal similarity. Second, I defend Descartes from the charge that his conception of similarity renders the condition trivial by making practically everything similar to everything else. Third, I discuss the familiar objection that if the similarity condition is not trivial, then it is incompatible with Cartesian mind–body interaction. I argue that the condition is consistent with, and indeed explains, Descartes's late view of interaction.