Mental substance is the idea held by dualists and idealists, that minds are made-up of non-physical substance. This substance is often referred to as consciousness.
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.
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.
Famous quotes containing the words mental and/or substance:
“Eccentricity has always abounded when and where strength of character has abounded; and the amount of eccentricity in a society has generally been proportional to the amount of genius, mental vigour, and moral courage which it contained.”
—John Stuart Mill (1806–1873)
“I do not remember anything which Confucius has said directly respecting man’s “origin, purpose, and destiny.” He was more practical than that. He is full of wisdom applied to human relations,—to the private life,—the family,—government, etc. It is remarkable that, according to his own account, the sum and substance of his teaching is, as you know, to do as you would be done by.”
—Henry David Thoreau (1817–1862)