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 describes his theory of mental substance (which he calls res cogitans) in the Second Meditation (II.8) and in Principia Philosophiae (2.002).