Catholic Dictionary




The theory that all physical beings are composed of two principles of being: the prime matter, which is an undefined primitive, and a substantial form, which is a definite mode of existence. The two principles are related to each other as potency and act. This theory, which St. Thomas adopted from Aristotle, is based on two features that characterize all material things: the changeableness and the different individuals of a species. Both features can be explained only on the supposition that material things in their deepest nature are composed of two ultimate elements: prime matter, which remains constant in any change and gives individuality to each being; and substantial form, which accounts for the actual existence of different things, their distinctive properties, and the profound alteration that takes place when one substance changes into another. (Etym. Greek hyl_, matter + morphe, contour, form.)