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This term dualism is used to describe any system in which there are two realities. The term is sometimes used to express the existence of two gods or the existence of God and the cosmos, but its most common usage is in the philosophy of human nature. A dualist holds that a human person is constituted by a body and what may be called a mind or soul or consciousness. Some dualists hold that persons are nonphysical concrete subjects who are embodied contingently. That is, a person may survive the destruction of his or her body, or one's body may continue to exist (as a corpse) after one has ceased to be. The greatest competing philosophy of human nature is materialism. While representatives of dualism in contemporary philosophy are in the minority, dualism is not easily uprooted philosophically, religiously, or culturally.

On the philosophical front, materialists often have difficulty capturing the evident existence of consciousness or felt experiences. Human thinking, sensing, and feeling appear to be different in kind from brain processes and other bodily activity. At a minimum, there is a profound causal relation between the two (one's thinking is contingent on neurological events), and yet a causal relation is not the same thing as identity. The mental and physical may be causally interdependent without being identical. Since 1980, a range of philosophers who are materialists either in their convictions or inclinations (e.g., Thomas Nagel, Colin McGinn, Jaegwon Kim, and John Pollock), have insisted that there are serious problems with identifying consciousness with physical states and processes.

A shift in contemporary science has also bolstered the case for dualism. So long as a strictly deterministic physical science dominated the view of nature, it appeared that something nonphysical (states of consciousness or the soul) would have no causal role in explaining events in the world. This would render a dualist account of action absurd. But quantum mechanics has advanced an indeterminist view of the cosmos, and it is more difficult to rule out dualism.

From a religious point of view, dualism is in play with most but not all traditions that acknowledge an afterlife. Some religions believe in a resurrection of the dead in which a person survives death by their material body being either reconstituted or re-created. But even these religions often preserve some immaterial locus or referent to secure a person's identity; in between physical death and resurrection a person might still be thought of as present to God. Virtually all religions that include a belief in reincarnation allow that there is some immaterial aspect to a person's or a soul's identity. If persons are identical with their bodies, then what happens to persons and bodies are the very same; dualism allows persons and souls to share a different fate from their bodies.

Dualism also receives some support from cultures that routinely adopt different methods for studying and talking about persons as opposed to studying and talking about their bodies. Consider a modest example in English: It can make sense to say that someone is in class but that his or her mind is far away.

History of the concept

Historically, the ancient Greek philosopher Plato (428–347 B.C.E.) was a key advocate of a form of dualism. Dualism is integral to his case for the immortality of the soul, as expressed in the Phaedrus, Phaedo, and Republic. Plato posited not just a postmortem existence but life before material embodiment (prenatal existence). Plato thought of a person's material embodiment as good but also as something that impedes the soul's longing for the good, the true, and the beautiful. Compared with the beauty and glory of disembodied life, material existence can be like a prison. The early Christian leader Augustine of Hippo (354–430 C.E.) developed a Platonic form of Christianity, rejecting some of Plato's beliefs (Augustine rejected pre-natal existence, as well as Plato's view of the divine as a finite reality) but preserving his dualism and the centrality of the good.

Some Platonic Christians in the medieval period speculated that God creates a host of various forms of intelligence in either embodied or disembodied form. This formed part of the principle of plentitude in medieval thought. The philosopher Thomas Aquinas (1225–1274) preserved much of the Platonic, Augustinian tradition but he more firmly insisted that human beings are comprised of matter and form. He still allowed that a person's soul persists after death, so Aquinas's reservations about radical dualism were limited.

Modern philosophy in Europe focussed on three philosophies of human nature. Dualism was championed by René Descartes (1596–1650); Cartesian dualism was advanced based on the conceivability of the self without the body. Thomas Hobbes (1588–1679) was very much on the other side. According to Hobbes, only matter exists and the very notion of there being something immaterial was nonsense. Hobbes insisted that even God is a material reality. A third position was championed by George Berkeley (1685–1753) who held that matter was not a fundamental, mind-independent reality. The cosmos is made up of minds and their sensory experiences. Berkeley's thesis that only minds and their states and activities exist is called idealism. In the eighteenth century it was possible to see dualism as a mediating, moderate choice between the extremes of materialism and idealism.

Many contemporary Christian theologians see dualism as part of an undesirable body-hatred; dualism is accused of foisting on people an excessively fragmented view of embodiment. Moreover, dualism is thought to reflect a vain attempt by humans to distinguish themselves from the rest of creation. These objections all seem answerable. There is no necessity for dualists to see embodiment in negative terms. And while a person's psychological and physical life can be fragmented, there is no need for dualism to regard human embodiment as always laden with bifurcation. Dualists may see the embodied person as a functional unity. As for the question of human pride, Descartes famously denied nonhuman animals were like humans in possessing (or being) minds. Descartes read nature in mechanical terms while he tried to secure an exception for human life. But most contemporary dualists see the emergence of consciousness as something involving nonhuman animal life; people share with some nonhumans in having experiences and possessing psychological abilities. Dualists tend to see the emergence of consciousness as something that prevails throughout the animal world and not something limited exclusively to human beings.

See also AUGUSTINE; EMBODIMENT; MATERIALISM; MIND-BODY THEORIES; MIND-BRAIN INTERACTION; MONISM; PLATO; THOMAS AQUINAS

CHARLES TALIAFERRO