This is opposed to the materialists, who hold that what we normally think of as mental substance is ultimately physical matter (i.e., brains).
For many philosophers, this word or the phrase "mental substance" has a special meaning.
Gottfried Leibniz, belonging to the generation immediately after Descartes, held the position that the mental world was built up by monads, mental objects that are not part of the physical world (see Monadology).
According to Descartes, God first created eternal truths and then the world from nothing, governing it with His divine providence.
Soul and body are in turn two finite modes of Thought and Extension.