DUALISM, the philosophical opinion that the ultimate components of the universe are two in number, or fall into two fundamentally dis: tinct classes. In modern times the line of cleavage between the halves of the dualistic world has usually separated the mental from the material. The 17th century view of the uni verse under the aspect of substance naturally caused dualism to assume the form of a belief in the existence of just two fundamental sub stances, mind and matter. This was the opinion of Descartes and of Locke. In the rationalistic school of the Continent as well as the empiricist school of Britain, it soon became obvious that the correspondences between the worlds of mind and of matter, as exhibited in experience and conduct, were not subject to an easy explanation on the basis of the existing type of dualism. Accordingly the continental rationalism was either forced, as in occasionalism, to fall back upon the honesty and the continual intervention of God, or else, as in the philosophies of Spinoza and Leibnitz, to discard dualism altogether. The British school similarly progressed out of dual ism, first by Berkeley's denial of the substantial character of matter, and later by Hume's denial of the substantial character of mind. A partial recrudescence of the dualism of substance is to be found in Kant's philosophy of things —in themselves. At present a substantive view of the mind is characteristic of the dualism of Wil liam McDougall. However, it may be said in general that since the 18th century the notion of substance has come to play a progressively smaller part in the question of monism and dual ism. The modern formulation of the question is: Are there two mutually exclusive orders of being, the one mental, the other material? While the agreement on this point is not by any means complete, there is a strong and growing tend ency, not only among those of an absolute idealist trend of thought, but also among rela tivists such as William James and realists such as Holt and Perry, to regard the elements en tering into the combinations of the material world and the components of mental states as not necessarily different. Mind and matter, in a philosophy of this type, are complexes of the same subject matter, but are viewed from dif ferent aspects or are ordered in a different manner.
The most interesting form of latter-day dualism is that of Bergson (q.v.). Bergson's
philosophy is based on a sharp contrast between the mental, possessing the continuity of memory. which allows the present as it were to contain the past, and the material, the subject-matter of physical science, forming a kinematographic succession of spatial arrangements of particles. In his earlier writings, the distinctness of these two worlds is emphatically asserted, but more recently he has come to regard matter as an arrested, atrophied manifestation of the same vital impulse that constitutes life and mind. Bergson thus forsakes dualism for monism.
Dualism is not necessarily a cleavage along the mind-matter plane. Accordingly we find that whatsoever manifestations of dualism exist in ancient philosophy depend on an en tirely different principle. This principle is the distinction between form and matter. The first crude attempts at cosmology on the part of the Ionian School brought everything back to one fundamental principle— water or air or fire or the unbounded, as the case might be. An axagoras made the first step toward dualism by making reason —albeit a reason or order but imperfectly distinguished from the objects ordered — the principle of the universe. Plato carried this dualistic trend to its logical con sequences, and distinguished between the ideas, representing order and form, and the evil prin ciple of not being ordered and shaped by them. While Aristotle did not directly entity his forms, he made such a sharp distinction be tween form and matter that he may justly be called a dualist. This dualism was accentuated in the excessive formalism of the scholastics.
Dualism is found in ethics as well as in ontology. Its ethical aspect is the recognition of separate fundamental principles of good and evil. It is especially characteristic of Zoroas trianism, and elsewhere is usually a borrowing from Persian sources. The strife between Or muzd and Ahriman becomes in Christianity the strife between good and Satan. Manichreanism, which was peculiarly under Zoroastrian influence had this dualism as its keynote, and Satan was not so subordinated to God as in the more nor mal forms of Christianity. (See MONISM). Con sult McDougall, (Body and Mind' (New York 1911), and the bibliographies to METAPHYSICS and PHILOSOPHY.