Mind–body dualism
Ontological dualism makes dual commitments about the nature of existence as it relates to mind and matter, and can be divided into three different types:Substance dualism is a type of dualism most famously defended by René Descartes, which states that there are two kinds of foundation: mental and body. This philosophy states that the mental can exist outside of the body, and the body cannot think. Substance dualism is important historically for having given rise to much thought regarding the famous mind–body problem. Substance dualism is a philosophical position compatible with most theologies which claim that immortal souls occupy an independent realm of existence distinct from that of the physical world.
Mortimer J. Adler
Adler was a self-proclaimed "moderate dualist", and viewed the positions of psychophysical dualism and materialistic monism to be opposite sides of two extremes. Regarding dualism, he dismissed the extreme form of dualism that stemmed from such philosophers as Plato (body and soul) and Descartes (mind and matter), as well as the theory of extreme monism and the mind–brain identity theory. After eliminating the extremes, Adler subscribed to a more moderate form of dualism. He believed that the brain is only a necessary, but not a sufficient, condition for conceptual thought; that an "immaterial intellect" is also requisite as a condition; and that the difference between human and animal behavior is a radical difference in kind. Adler defended this position against many challenges to dualistic theories.