Mental substance
Mental substance can refer to the concept that dualists and idealists hold, that minds are made-up of non-physical substance.
This is as opposed to materialists, who hold that what we normally think of as mental substance is ultimately physical stuff (i.e., brains).
Descartes, who was famous for such assertions as "I think therefore I am", which is a kind of popular summary of some of the things that he said, has had a lot of influence on the thinking around the mind-body problem.
He used a more precise definition of the word "substance" than is currently popular. It was that a substance is that which can exist without the existence of any other substance. So, for many philosophers, this word or the phrase "mental substance" has a special meaning in this area.
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 also
