Mental substance

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.