It is common to see matter as a type of substance, that is, as an entity that possesses features such as shape or mass, as a sort of thing distinct in kind from mind or soul or spirit. Aristotle held that matter, or rather prime matter—the most basic stuff out of which material substances are made—is not a substance but a “principle of potency” that needs to be actualized by what he called substantial form in order to constitute, with form, a complete natural body such as a stone, a tree, or a cat. Medieval Aristotelians appropriated, in various ways, Aristotle’s basic teaching that matter is not a substance. St. Thomas Aquinas opposed the thesis that all creatures are partially constituted by matter; in particular, he thought that the angels, being pure spirits, are pure forms without any matter. Aquinas did not define substance as that which has accidents—he defined it as something that exists “in itself,” not in another.
The Routledge Companion to Medieval Philosophy
John Kronen and Sandra Menssen. “Matter.” In The Routledge Companion to Medieval Philosophy, 79-85, 2021.