Belief that the mental things (which we refer to with words like "mind" or "consciousness" or "self" or "soul" or "spirit") and physical things (which we refer to with words like "body" or "material object" or simply "matter") are fundamentally distinct kinds of entities, or in the language a classical metaphysics, "substances" (Aristotle's ousia, or "beings," traditionally mistranslated into the Latin "substantia").  "Bodies" are "substances" possessing physical properties such as the "primary" properties of extension, while "minds" are regarded as "substances" possessing mental properties, such as thinking, judging, believing, willing, etc. As a solution  to the traditional mind-body problem, dualism derives especially from Descartes and his followers in the seventeenth century.

        Variations on this dualist theme (including three subvarieties of dualism: interactionism, parallelism, and epiphenomenalism) arise when dualists try to explain why events in the supposedly separate realms of mind and body seem so well-coordinated with each other.

        Both materiallism and idealism are ontological responses to try to get out of the problems posed by Descartes' dualistic metaphysics.