Mental substance, according to the idea held bydualists andidealists, is a non-physical substance of which minds are composed. This substance is often referred to asconsciousness.
This is opposed to the view ofmaterialists, who hold that what we normally think of as mental substance is ultimately physical matter (i.e., brains).
René Descartes, who was most famous for the assertion "I think therefore I am", played a major role in developing themind–body problem. He describes his theory of mental substance (which he callsres cogitans distinguishing it from theres extensa) in theSecondMeditation (II.8) and inPrincipia Philosophiae (2.002).
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.
According to Descartes, God first created eternal truths and then the world from nothing, governing it with Hisdivine providence. He took special care of human creatures, placing innate ideas in their thought, starting with the ideas ofperfection andinfinity.[1]
Gottfried Leibniz, belonging to the generation immediately after Descartes, held the position that the mental world was built up ofmonads, mental objects that are not part of the physical world (seeMonadology).
The distinction betweenres cogitans andres extensa was taken up inSpinoza'sEthics, written between 1661 and 1675, according to which Thought andExtension are two infinite attributes of the one divine Substance. Soul and body are in turn two finite modes of Thought and Extension.
.png)