Mental substance

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Mental substance, according to the idea held by dualists and idealists, is a non-physical substance of which minds are composed. This substance is often referred to as consciousness. This is opposed to the view of materialists, 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 the mind–body problem. He describes his theory of mental substance (which he calls res cogitans distinguishing it from the res extensa) in the Second Meditation (II.8) and in Principia 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. Descartes sought a foundation of knowledge that could not be doubted. He employed a method of radical doubt, questioning all beliefs that could possibly be uncertain. However, Descartes realized that the very act of doubting presupposed a thinking subject. If he was doubting, then he must be thinking; and if he was thinking, then he must exist as a thinking being. This insight led to his famous conclusion cogito, ergo sum ("I think, therefore I am").