Dualism (from Lat. duo, two), like most other philosophical terms, has been employed in different meanings by different schools. First, the name has been used to denote the religious or theological system which would explain the universe as the outcome of two eternally opposed and coexisting principles, conceived as good and evil, light and darkness, or some other form of conflicting powers. We find this theory widely prevalent in the East, and especially in Persia, for several centuries before the Christian Era. The Zend-Avesta, ascribed to Zoroaster, who probably lived in the sixth century B.C. and is supposed to be the founder or reformer of the Medo-Persian religion, explains the world as the outcome of the struggle between Ormuzd and Ahriman. Ormuzd is infinite light, supreme wisdom, and the author of all good; Ahriman is the principle of darkness and of all evil. In the third century after Christ, Manes, for a time a convert to Christianity, developed a form of Gnosticism, subsequently styled Manichaeism, in which he sought to fuse some of the elements of the Christian religion with the dualistic creed of Zoroastrianism (see Manichaeism and Zoroaster). Christian philosophy, expounded with minor differences by theologians and philosophers from St. Augustine downwards, holds generally that physical evil is the result of the necessary limitations of finite created beings, and that moral evil, which alone is evil in the true sense, is a consequence of the creation of beings possessed of free wills, and is tolerated by God. Both physical and moral evil are to be conceived as some form of privation or defect of being, not as a positive entity. Their existence is thus not irreconcilable with the doctrine of theistic monism. Second, the term dualism is employed in opposition to monism, to signify the ordinary view that the existing universe contains two radically distinct kinds of being or substance, matter and spirit, body and mind. This is the most frequent use of the name in modern philosophy, where it is commonly contrasted with monism. But it should not be forgotten that dualism in this sense is quite reconcilable with a monistic origin of all things. The theistic doctrine of creation gives a monistic account of the universe in this sense. Dualism is thus opposed to both materialism and idealism. Idealism, however, of the Berkeleyan type, which maintains the existence of a multitude of distinct substantial minds, may, along with dualism, be described as pluralism.
Historically, in Greek philosophy as early as 500 B.C. we find the Eleatic School with Parmenides as their chief, teaching a universal unity of being, thus exhibiting a certain affinity with modern German monism. Being alone exists. It is absolutely one, eternal, and unchangeable. There is no real becoming or beginning of being. Seeming changes and plurality of beings are mere appearances. To this unity of being Plato opposed an original duality God and unproduced matter, existing side by side from all eternity. This matter, however, was conceived as indeterminate, chaotic, fluctuating, and governed by a blind necessity, in contrast with mind which acts according to plan. The order and arrangement are due to God. Evil and disorder in the world have their source in the resistance of matter which God has not altogether vanquished. Here we seem to have a trace of the Oriental speculation. Again there is another dualism in man. The rational soul is a spiritual substance distinct from the body within which it dwells, somewhat as the charioteer in the chariot. Aristotle is dualistic on sundry important topics. The contrast between the fundamental conceptions of matter and form of a potential and an actualizing principle runs through all branches of his system. Necessarily coeternal with God, Who is pure actuality, there has existed the passive principle of matter, which in this sense, however, is mere potentiality. But further, along with God Who is the Prime Mover, there must also have existed from all eternity the World moved by God. In his treatment of cognition Aristotle adopts the ordinary common-sense view of the existence of individual objects distinct from our perceptions and ideas of them. Man is an individual substantial being resulting from the coalescence of the two principles form (the soul) and matter.
Christianity rejected all forms of a dual origin of the world which erected matter, or evil, or any other principle into a second eternal being coexistent with God; and it taught the monistic origin of the universe from one, infinite, self-existing spiritual Being who freely created all things. The unfamiliar conception of free creation, however, met with considerable opposition in the schools of philosophy and was abandoned by several of the earlier heresies. The neo-Platonists sought to lessen the difficulty by emanastic forms of pantheism, and also by inserting intermediate beings between God and the world. But the former method implied a materialistic conception of God, while the latter only postponed the difficulty. From the thirteenth century, through the influence of Albertus Magnus and still more of St. Thomas Aquinas, the philosophy of Aristotle, though subjected to some important modifications, became the accredited philosophy of the Church. The dualistic hypothesis of an eternal world existing side by side with God was of course rejected. But the conception of spiritual being as opposed to matter received fuller definition and development. The distinction between the human soul and the body which it animates was made clearer and their separability emphasized; but the ultra-dualism of Plato was avoided by insisting on the intimate union of soul and body to constitute one substantial being under the conception of form and matter.
The problem of dualism, however, was lifted into quite a new position in modern philosophy by Descartes (q.v.). Indeed, since his time it has been a topic of central interest in philosophical speculation. His handling of two distinct questions, the one epistemological, the other metaphysical, brought this about. The mind stands in a cognitional relation to the external world, and in a causal relation to the changes within the body. What is the precise nature of each of these relations? According to Descartes the soul is res cogitans. Its essence is thought. It is simple and unextended. It has nothing in common with the body, but is connected with it in a single point, the pineal gland in the center of the brain. In contrast with this, the essence of matter lies in extension. So the two forms of being are utterly disparate. Consequently the union between them is of an accidental or extrinsic character. Descartes thus approximates to the Platonic conception of charioteer and chariot. Soul and body are really two merely allied beings. How then do they interact? Real reciprocal influence or causal interaction seems impossible between such disparate things. Geulincx and other disciples of Descartes were driven to invent the hypothesis of occasionalism and Divine assistance, according to which it is God Himself who effects the appropriate change in either body or mind on the occasion of the corresponding change in the other. For this system of miraculous interferences Leibniz substituted the theory of pre-established harmony according to which God has coupled pairs of bodies and souls which are destined to run in parallel series of changes like two clocks started together. The same insoluble difficulty of psycho-physical parallelism remains on the hands of those psychologists and philosophers at the present day who reject the doctrine of the soul as a real being capable of acting on the body which it informs. The ultra-dualism of Descartes was immediately followed on the Continent by the pantheistic monism of Spinoza, which identified mind and matter in one infinite substance of which they are merely “modes”.
The cognitional question Descartes solves by a theory of knowledge according to which the mind immediately perceives only its own ideas or modifications. The belief in an external world corresponding to these ideas is of the nature of an inference, and the guaranteeing of this inference or the construction of a reliable bridge from the subjective world of thought to the objective world of material being, was thenceforth the main problem of modern philosophy. Locke similarly taught that the mind immediately apprehends only its own ideas, but he assumed a real external world which corresponds to these ideas, at least as regards the primary qualities of matter. Berkeley, accepting Locke’s assumption that the mind immediately cognizes only its own ideas, raised the question: What grounds have we for believing in the existence of a material world corresponding to those ideas? He concludes there are none. The external cause of these ideas is God Who awakens them in our minds by regular laws. The dualistic opposition between mind and matter is thus got rid of by denying an independent material world. But Berkeley still postulates a multitude of real substantial minds distinct from each other and apparently from God. We have thus idealistic pluralism. Hume carried Berkeley’s scepticism a step farther and denied the existence of permanent spiritual substances, or minds, for grounds similar to those on which Berkeley rejected material substances. All we know to exist are ideas of greater or less vividness. Kant repudiates this more extreme scepticism and adopts, at least in the second edition of his chief work, a form of dualism based on the distinction of phenomena and noumena. The mind immediately perceives only its own representations. These are modified by innate mental forms. They present to us only phenomena. But the noumena, the things in themselves, the external causes of these phenomenal representations, are beyond our power of cognition. Fichte rejected things-in-themselves outside the mind, and reduced the Kantian dualism to idealistic monism. The strongest and most consistent defenders of dualism in modern philosophy have been the Scotch School, including Reid, Stuart, and Hamilton. Among English writers in more recent times Martineau, McCosh, Mivart, and Case have carried on the same tradition on similar lines.
The problem of dualism, as its history suggests, involves two main questions: (I) Does there exist a material world outside of our minds and independent of our thought? (2) Supposing such a world to exist, how does the mind attain to the cognition of it?—The former question belongs to epistemology, material logic, or general philosophy; the latter to psychology. It is true that dualism is ultimately rejected by the materialist who reduces conscious states to functions or “aspects” of the brain; but objections from this standpoint will be more suitably dealt with under materialism and monism. The idealist theory since Berkeley, in all its forms, maintains that the mind can only know its own states or representations, and that what we suppose to be an independent material world is, in the last analysis, only a series of ideas and sensations plus belief in the possibility of other sensations. Our conviction of the objective reality of a vivid consistent dream is analogous to our conviction of the validity of our waking experience. Dualism affirms, in opposition to all forms of idealism, the independent extramental reality of the material world. Among its chief arguments are the following: (I) Our belief in the existence of other minds is an inference from their bodies. Consequently the denial of an external material world involves the rejection of all evidence for the existence of other minds, and lands the idealist in the position of “Solipsism”. (2) Physical science assumes the existence of a material world, existing when unperceived, possessing various properties, and exerting various powers according to definite constant laws. Thus astronomy describes the movements of heavenly bodies moving in ‘space of three dimensions, attracting each other with forces inversely proportioned to the square of the distance. It postulates the movement and action of such bodies when they are invisible as well as when they are visible through long periods of time and over vast areas of space. From these assumptions it deduces future positions and foretells eclipses and transits many years ahead. Observations carried out by subsequent generations verify the predictions. Were there not an extramental world whose parts exist and act in a space and time truly mirrored by our cognitions and ideas, such a result would be impossible. The branches of science dealing with sound, light, heat, and electricity are equally irreconcilable with idealism. (3) The teachings of physiology and psycho-physics become peculiarly absurd in the idealist theory. What, for instance, is meant by saying that memory is dependent on modifications in the nervous substance of the brain, if all the material world, including the brain, is but a collection of mental states? (4) Psychology similarly assumes the extramental reality of the human body in its account of the growth of the senses and the development of perception. Were the idealist hypothesis true its language would be meaningless. All branches of science thus presuppose and confirm the dualistic view of common sense.
Granted, then, the truth of dualism, the psychological question emerges: How does the mind come to know the material world?—Broadly speaking there are two answers. According to one the mind immediately perceives only its own representations or ideas and from these it infers external material objects as the cause of these ideas. According to the other, in some of its acts it immediately perceives extended objects or part of the material world. As Hamilton says: “What we directly apprehend is the Non-ego, not some modification of the Ego”. The theory which maintains an immediate perception of the non-ego he calls natural dualism or natural realism. The other, which holds a mediate cognition of the non-ego, as the inferred cause of a representation immediately apprehended, he terms hypothetical dualism or hypothetical realism. The doctrine of immediate or presentative perception is that adopted by the great body of the Scholastic philosophers and is embodied in the dictum that the idea, concept, or mental act of apprehension is non id quod percipitur sed medium quo res percipitur—not that which is perceived but the medium by which the object itself is perceived. This seems to be the only account of the nature of knowledge that does not lead logically to idealism; and the history of the subject confirms this view. But affirmation of the mind’s capacity for immediate perception of the non-ego and insistence on the distinction between id quod and id quo percipitur, do not dispose of the whole difficulty. Modern psychology has become genetic. Its interest centers in tracing the growth and development of cognition from the simplest and most elementary sensations of infancy. Analysis of the perceptive processes of a later age, e.g. apprehension of size, shape, solidity, distance, and other qualities of remote objects, proves that operations seemingly instantaneous and immediate may involve the activity of memory, imagination, judgment, reasoning, and subconscious contributions from the past experience of other senses. There is thus much that is indirect and inferential in nearly all the percipient acts of mature life. This should be frankly admitted by the defender of natural dualism, and the chief psychological problem for him at the present day is to sift and discriminate what is immediate and direct from what is mediate or representative in the admittedly complex cognitional operations of normal adult life.