DUALISM (from rare Lat. dualis, containing two, from duo), a philosophical term applied to all theories which attempt to explain facts by reference to two coexistent principles. The term plays an important part in metaphysical, ethical and theological speculation.
In Metaphysics. - Metaphysical dualism postulates the eternal coexistence of mind and matter, as opposed to monism both idealistic and materialistic. Two forms of this dualism are held. On the one hand it is said that mind and matter are absolutely heterogeneous, and, therefore, that any causal relation between them is ex hypothesi impossible. On the other hand is a hypothetical dualism, according to which it is held that mind cannot bridge over the chasm so far as to know matter in itself, though it is compelled by its own laws of cause and effect to postulate matter as the origin, if not the motive cause, of its sensations. It follows that, for the thinking mind, matter is a necessary hypothesis. Hence the theory is a kind of monism, inasmuch as it confessedly does not assert the existence of matter save as an intellectual postulate for the thinking mind. Matter, in other words, must be assumed to exist, though mind cannot know it in itself. From this question there emerges a second and more difficult problem. Consciousness, it is held, is of two main kinds, sensation and reason. Sensation alone is insufficient to explain all our intellectual phenomena; all sensation is momentary and individual (cf. Empiricism). How then are we to account for memory and the principles of necessity, similarity, universality? It is argued that there must be in the mind an enduring, primary faculty whereby we retain, compare and group the presentations of sense. This faculty is a priori, transcendental, and entirely separate from all the data of experience and sense-perception. Here then we have a dualism within experience. The mind is not to be regarded as a sensitized film which automatically records the impressions of the senses. It contains within itself this modifying critical faculty which reacts upon and arranges the sense-given presentations.
In Ethics and Theology. - In the domain of morals, dualism postulates the separate existence of Good and Evil, as principles of existence. In theology the appearance of dualism is sporadic and has not the fundamental, determining importance which it has in metaphysics. It is a result rather than a starting-point. The old Zoroastrianism, and those Christian sects (e.g. Manichaeism) which were influenced by it, postulate two contending deities Ormuzd and Ahriman (Good and Evil), which war against one another in influencing the conduct of men. So, in Christianity, the existence of Satan as an evil influence, antagonistic to God, involves a kind of dualism. But generally speaking this dualism is permissive, inasmuch as it is always held that God will triumph over Satan in His own time. So in Zoroastrianism the dualism is not ultimate, for Ahriman and Ormuzd are represented as the twin sons of Zervana Akarana, i.e. limitless time, wherein both will be finally absorbed. The postulate of an Evil Being arises from the difficulty, at all times acutely felt by a certain type of mind, of reconciling the existence of evil with the divine attributes of perfect goodness, full knowledge and infinite power. John Stuart Mill (Essay on Religion) preferred to disbelieve in the omnipotence of God rather than forgo the belief in His goodness. It follows from such a view that Satan is not the creation of God, but rather a power coeval in origin, over whose activity God has no absolute control.
In Theology. - Dualism is also used in a special theological sense to describe a doctrine of the Nestorian heresy. According to this doctrine the personality of Christ is twofold; the divine Logos dwells as a distinct personality in the man Jesus Christ, the union of the two natures being analogous to the relation between the believer and the indwelling Holy Spirit.
History of the Doctrine. - The earliest European thinkers (see Ionian School of Philosophy) endeavoured to reduce all the facts of the universe to a single material origin, such as Fire, Water, Air. It is only gradually that there appears any recognition of a spiritual principle exercising a modifying or causal influence over inert matter. Anaxagoras was the first to postulate the existence of Reason as the source of change and progress. Yet even he did not conceive this Reason as incorporeal; it was in reality only the most highly rarefied form of matter in existence. In Plato for the first time we find a truly dualistic conception of the universe. Asserting that Ideas alone really exist, he yet found it necessary to postulate a second principle of not-being, the groundwork of sensuous existence and of imperfection and evil. Herein he identified metaphysics and ethics, combining the good with the truly existent and evil with the non-existent. Aristotle rebels against this conception and substitutes the idea of and development. Nevertheless he does not escape from the dualism of Form and Matter, and . The scholastic philosophers naturally held dualistic views resulting from their extreme devotion to formalism. This blind dualism found its natural consequence in the revolt of the Renaissance thinkers, Bruno and Paracelsus, who asserted the unity of mind and matter in all existence and were the precursors of the more intelligent monism of Leibnitz and the scientific metaphysics of his successors. The birth of modern physical science on the other hand in the investigations of Bacon and Descartes obscured the metaphysical issue by the predominance of the mechanical principles of natural philosophy. They attempted to explain the fundamental problems of existence by the unaided evidence of the new natural science. Thus Descartes maintained the absolute dualism of the res cogitans and the res extensa. Spinoza realized the flaw in the division and preferred to postulate behind mind and matter a single substance (unica substantia) while Leibnitz explained the universe as a harmony of spiritual or semispiritual principles. Kant practically abandons the problem. He never really establishes a relation between pure reason and things-in-themselves (Dinge an sich), but rather seeks refuge in a dualism within consciousness, the transcendental and the empirical. Since Kant there are, therefore, two streams of dualism, dealing, one with the radical problem of the relation between mind and matter, the other with the relation between the pure rational and the empirical elements within consciousness. To the first problem there is one obvious and conclusive answer, namely that matter in itself is inherently unthinkable and comes within the vision of the mind only as an intellectual presentation. It follows that philosophy is in a sense both dualist and monist; it is a cosmic dualism inasmuch as it admits the possible existence of matter as a hypothesis, though it denies the possibility of any true knowledge of it, and is hence in regard of the only possible knowledge an idealistic monism. It is a self-destructive dualism, a confessedly one-sided monism, agnostic as to the fundamental problem. To the second problem there are two main answers, that of Associationism which denies to the mind any a priori existence and asserts that sensation is the only source of knowledge, and that which admits the existence of both transcendental and empirical knowledge.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)