Mental substance, according to the idea held by dualists and

idealists In philosophy, the term idealism identifies and describes metaphysical perspectives which assert that reality is indistinguishable and inseparable from perception and understanding; that reality is a mental construct closely connected to ...

, is a non-physical substance of which minds are composed. This substance is often referred to as

consciousness Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...

. This is opposed to the

materialists Materialism is a form of philosophical monism which holds matter to be the fundamental substance in nature, and all things, including mental states and consciousness, are results of material interactions. According to philosophical materialism ...

, 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 The mind–body problem is a philosophical debate concerning the relationship between thought and consciousness in the human mind, and the brain as part of the physical body. The debate goes beyond addressing the mere question of how mind and bo ...

. He describes his theory of mental substance (which he calls ''res cogitans'' distinguishing it from the ''

res extensa ''Res extensa'' is one of the two substances described by René Descartes in his Cartesian ontology (often referred to as "radical dualism"), alongside '' res cogitans''. Translated from Latin, "''res extensa''" means "extended thing" while th ...

'') in the ''Second

Meditation Meditation is a practice in which an individual uses a technique – such as mindfulness, or focusing the mind on a particular object, thought, or activity – to train attention and awareness, and achieve a mentally clear and emotionally calm ...

'' (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.

Gottfried Leibniz Gottfried Wilhelm (von) Leibniz . ( – 14 November 1716) was a German polymath active as a mathematician, philosopher, scientist and diplomat. He is one of the most prominent figures in both the history of philosophy and the history of mathem ...

, belonging to the generation immediately after Descartes, held the position that the mental world was built up by

monad Monad may refer to: Philosophy * Monad (philosophy), a term meaning "unit" **Monism, the concept of "one essence" in the metaphysical and theological theory ** Monad (Gnosticism), the most primal aspect of God in Gnosticism * ''Great Monad'', a ...

s, mental objects that are not part of the physical world (see ''

Monadology The ''Monadology'' (french: La Monadologie, 1714) is one of Gottfried Leibniz's best known works of his later philosophy. It is a short text which presents, in some 90 paragraphs, a metaphysics of simple substances, or '' monads''. Text Dur ...

'').

See also