Formal and Proper

Aquinas, following Aristotle, distinguishes between substances and their accidents: that is, between things and their attributes. He also distinguishes between proper accidents (also known as properties in Scholastic terminology) and accidental accidents, that is, between accidents that belong to a substance in virtue of what it is, and accidents that belong to it due to extrinsic factors. Aquinas says that a thing’s proper accidents are caused by the thing’s own essential principles. John Wippel interprets Aquinas as holding that a substance efficiently causes its own proper accidents. I argue that Aquinas is more plausibly read as holding that a substance formally causes its own proper accidents. Formal causality extends to more than just the informing of matter. Formal causality is a principle of determination: being a certain substance involves the determination of having certain proper accidents.