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:
“It was at a particular moment in the history of my own rages that I saw the Western world conditioned by the images of Marx, Darwin and Freud; and Marx, Darwin and Freud are the three most crashing bores of the Western world. The simplistic popularization of their ideas has thrust our world into a mental straitjacket from which we can only escape by the most anarchic violence.”
—William Golding (b. 1911)
“Ah, to build, to build!
That is the noblest art of all the arts.
Painting and sculpture are but images,
Are merely shadows cast by outward things
On stone or canvas, having in themselves
No separate existence. Architecture,
Existing in itself, and not in seeming
A something it is not, surpasses them
As substance shadow.”
—Henry Wadsworth Longfellow (1807–1882)