dualism

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The condition of being double; duality.
• noun Philosophy The view that the world consists of or is explicable as two fundamental entities, such as mind and matter.
• noun Psychology The view that the mind and body function separately, without interchange.
• noun The concept that the world is ruled by the antagonistic forces of good and evil.
• noun The concept that humans have two basic natures, the physical and the spiritual.

from The Century Dictionary.

• noun Division into two; a twofold division; duality.
• noun In philosophy, in general, that way of thinking which seeks to explain all sorts of phenomena by the assumption of two radically independent and absolute elements, without any continuous gradation between them: opposed to monism.
• noun To the doctrine of a double absolute, especially a principle of good and a principle of evil, or a male and a female principle.
• noun In theol.: The doctrine that there are two independent divine beings or eternal principles, one good and the other evil: characteristic especially of Parsism and various Gnostic systems.
• noun The heretical doctrine, attributed to Nestorius by his opponents, of the twofold personality of Christ, the divine logos dwelling as a separate and distinct person in the man Christ Jesus, and the union of the two natures being somewhat analogous to the indwelling of the Holy Spirit in the believer; that view of the personality of Christ which regards him as consisting of two personalities.
• noun In chem., a theory advanced by Berzelius which assumed that every compound, whether simple or complex, must be constituted of two parts of which one is positively and the other negatively electrified.
• noun In general, any system or theory involving a duality of principles

from the GNU version of the Collaborative International Dictionary of English.

• noun (Philos.), (Theol.) A view of man as constituted of two original and independent elements, as matter and spirit.
• noun A system which accepts two gods, or two original principles, one good and the other evil.
• noun The doctrine that all mankind are divided by the arbitrary decree of God, and in his eternal foreknowledge, into two classes, the elect and the reprobate.
• noun (Physiol.) The theory that each cerebral hemisphere acts independently of the other.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Duality; the condition of being double.
• noun philosophy The view that the world consists of, or is explicable in terms of, two fundamental principles, such as mind and matter or good and evil.
• noun theology The belief that the world is ruled by a pair of antagonistic forces, such as good and evil; the belief that man has two basic natures, the physical and the spiritual.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the doctrine that reality consists of two basic opposing elements, often taken to be mind and matter (or mind and body), or good and evil