This is opposed to the materialists, who hold that what we normally think of as mental substance is ultimately physical matter (i.e., brains).
Descartes, who was most famous for the assertion "I think therefore I am," has had a lot of influence on the mind-body problem.
He used a more precise definition of the word "substance" than is currently popular: that a substance is something which can exist without the existence of any other substance. For many philosophers, this word or the phrase "mental substance" has a special meaning.
Transcendental Epistemology and Proofs of the Existence of God in Seventeenth-Century Rationalist Philosophy: The Proof from the Existence of Mental Substance in Nicolas Malebranche's Search after Truth
Nov 01, 2013; "The science of man is not only estimable because it raises us above others; it is even more so because it lowers and humbles us...