Metaphysical problem of the relationship between mind and body.
The modern problem stems from the thought of dualism . Descartes's interactionism had many critics even in his own day. Thomas Hobbes denied the existence of mental substance. Materialism of a sort was also supported by Descartes's correspondent Pierre Gassendi (1592–1655). Benedict de Spinoza posited a single substance of which the mental and the material are attributes; his theory is known as psycho-physical parallelism. More recent views include the {{link=double-aspect theory">double-aspect theory , identity theory , eliminative materialism (which denies the reality of the familiar categories of mental state posited in so-called folk psychology ), and theories of supervenience .