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The name given to any theory, whether general or limited, that invokes two opposed and heterogeneous principles of explanation; as such, it differs from both monism and pluralism, at least when the latter involves more than two principles. The difficulties posed by the existence of evil, moral and physical, were the earliest source of dualistic theories. In modern times, dualism is espoused most frequently in dealing with the problem of knowledge; here thought and being, mind and body, certitude and opinion seem to be dyads whose members are irreducible the one to the other. The problem of knowledge was the source of some early dualistic theories as well.

Early Theories. From earliest times it has seemed to many that there is no acceptable way in which good and evil can be reduced to the same source. And, since both good and evil are found in the universe, the universe itself is not the product of one author. The evil in question can be physical, as, for example, defective structure of plant, animal, or human body, or it can be moral, the prevalence, temporary or permanent, of man's evil tendencies. Moral evil has, through the centuries, evoked the image of the human person as a battleground, a locus of conflicting tendencies, some good, others bad. A man can recognize and approve the right course and yet, as Ovid and St. Paul poignantly observe, pursue its opposite. Whence comes moral evil? What is the explanation of defective being? Whether one thinks of the moral question as a particularization of the physical or the physical as an extrapolation from the sense of moral evil, there is historically a link between the ethical and the ontological in dualism. The universe seems the result of a struggle between Good and Evil, Light and Darkness. Such dualism is to be found in Persia, in the teaching of zoroaster (zarathushtra). In Christian times this same dualism is expressed in manichaeism.

In its beginnings in Ionia, greek philosophy sought a monistic explanation of the world. Thales, Anaximander, and Anaximenes may be seen as postulating a single underlying source of the multiple things of ordinary experience when they suggested, respectively, that water, the unlimited, or air is nature; yet, because they were physical philosophers and took change seriously, they held that there are many things. It is this assumption of natural philosophy that was challenged by parmenides, the most uncompromising monist of antiquity. being is, nonbeing is not. This Parmenidean assertion entails the negation of change and multiplicity, for if being is said to come to be, it must come from either being or nonbeing. It cannot come from nonbeing; and, if from being, no change has occurred. Multiplicity is canceled out by the same logic. Given two things, they can differ in either being or nothing; but being is what they have in common, and if they differ in nothing, they are the same. This monism, then, is thorough: the one, being, is not the source of any later multiplicity (see unity). But if the doctrine of Parmenides must be called monistic, the seeds of dualism were present in the poem in which the great Eleatic set down his thought. The monistic doctrine just sketched is found in that part of the poem called the way of truth; a second part, the way of opinion, speaks of the world as it appears to man, and Parmenides there gives a cosmological doctrine not unlike those of his predecessors. This bifurcation between what really and truly is, on the one hand, and what appears, on the other, was to be developed by Plato; before turning to that, however, mention must be made of Pythagoreanism. pythagoras thought of physical things as numbers; the principles of number, the odd and even, were the principles of all things. This fundamental dualism led to a list of opposites, the so-called Pythagorean categories (see Aristotle, Meta. 985b 23986b 1).

Platonic Dualism. Platonic dualism arose from Parmenides and Pythagoreanism; its motivations were at once ethical and epistemic. plato was struck by the nonempirical character of ethical ideals; for example, one need not encounter a perfect instance of justice in order to desire to be just; rather man judges some acts to be just because he recognizes in them an imitation of the idea of justice. So too, since man has never experienced perfect equality between physical things, his notion of equality cannot be derived from experience. Justice, equality, and gradually other natures come to be looked upon as ideals that phenomenal things strive to imitate. Add to this the recognition that true knowledge implies an unchanging object, an object unaffected by time, and one has two motives for Plato's assertion that above and beyond the material world where things imitate ideal natures and, being forever in flux, cannot be objects of true knowledge, there is another and better world, the world of Ideas. The Ideas are the subsistent ideals that phenomenal things imitate; they are the guarantee of true knowledge. Plato thus introduced a radical dualism. Only Ideas truly are, but the things of this world have an extenuated kind of being. Knowledge is of Ideas; phenomenal things can ground only opinion.

Aristotelian Dualism. Aristotle, Plato's pupil and colleague, thought that such arguments as Plato himself had formulated in the Parmenides were conclusive against the Ideas: the whole doctrine was a great mistake. Thus, in Aristotle's view, if there are things existing separately and apart from the material world, a better argument for their existence would have to be devised. In his famous analysis of moved movers, Aristotle proved that such movers demand a mover that is itself unmoved (see motion, first cause of). This First Mover was then seen to have other attributes that reveal His being as personal; Christians have always and rightly observed that here Aristotle had in effect proved the existence of God.

The status of matter in Aristotle's philosophy raises the question whether his worldview is monistic or dualistic. That is, is everything in the cosmos reduced to the First Mover as to its cause, or is there something in the world that enjoys existence apart from the causality of the First Mover? Aristotle held that, so far as man can know, the world has always existed; thus motion has always existed and matter as well. The First Mover is pure act; matter, what Aristotle called primary matter, is pure potency. Does one have here two irreducibly different principles of the world man knows? It is important to see that this consequence does not follow because, in seeing so, one is better able to appreciate the difference between Aristotelian and Cartesian matter.

Aristotle's analysis of physical things, that is, of things that come to be as a term of a change, led him to maintain that physical things are composed of matter and form (see matter and form). Roughly, matter is what survives a change and form is what the matter gains as the result of the change. These principles are not themselves things; that is, matter cannot exist except in material things, and form exists only in formed things. If matter exists only in material things, it is not an entity on the same level as the First Mover. A similar opposition in Aristotelian philosophy, it may be noted, is that between body and soul, particularly when one restricts the consideration to the soul whose operations are not bound up with matter (see soul, human). Matter versus form, body versus human soul, material substance versus immaterial substancethese are the undeniable dualisms in Aristotle's doctrine.

Whether everything in the cosmos can be reduced to one principle is a question whose answer must be formulated carefully. Aristotle is quite clear as to the various meanings he ascribes to the term principle. It is commonly agreed among scholars that he taught that the First Mover is the ultimate final cause of everything in the cosmos, the good toward which all things tend. Some scholars have thought they could accept this conclusion and still doubt whether, for Aristotle, the First Mover is as well the efficient cause of everything. Against this doubt it can be argued that, unless there is an efficient causality comprehensive enough to fashion everything so that its telos is the First Mover, the order of the universe would be, on Aristotelian grounds, due to pure chance. But this Aristotle explicitly denies. The sense of the Aristotelian view of the universe, then, would be that there is one principle on whom depend Earth and the heavens and everything therein. Therefore, all dualisms in Aristotle are, so to speak, regional and not universal.

Scholastic Dualism. In Christian times the most important new candidate for the title of dualism would be the contrast between nature and grace, between the natural and the supernatural. From a metaphysical point of view, however, the most important clarification of the difference between God and creature was had in the doctrine of essence and existence. The sources of this distinction can be found in the Greeks, but the document that forced subsequent discussion of it was the De hebdomadibus of boethius. Among the later scholastics who commented on this opusculum was St. thomas aquinas. "Diversum est esse et id quod est," Boethius wrote, and St. Thomas, combining this with the Aristotelian doctrine of matter and form, understood its import as follows: "In substances composed of matter and form, there is a twofold composition of act and potency, the first of the substance itself that is composed of matter and form, the second of the already constituted substance and its existence (esse ), which can also be said to be a composition resulting from what it is (quod est ) and its existence (esse ), or from 'that which is' (quod est ) and 'that whereby it is' (quo est ). Thus it is clear that the composition of act and potency is found in more things than the composition of form and matter. Form and matter divide natural substance but potency and act divide being in general" (C. gent. 2.54). Only in God is there no composition whatsoever. He is being essentially; everything else is being by participation. As Ipsum Esse Subsistens, God is the source of the essence and existence of all creatures. Thus, if all created being is divided by potency and act at least in the sense of essence and existence, the source of all created being is one, God, whose essence is His existence (see essence and existence; po tency and act).

Cartesian Dualism. The division of reality into thought and extension is already implied in the methodical doubt that characterizes the philosophy of R. des cartes. In his effort to find an indubitable starting point, Descartes agreed to set aside anything about which doubt was at all possible. Sensible things, those of which man is aware by sensation, were first set aside, since it is in principle possible to think that one is deceived concerning shape, size, colors, etc. Man may even be deceived about his own body. But if any item of knowledge that might be substituted for X in "I think that X " can be thought of as possibly false, and therefore dubitable, man cannot doubt the existence of himself as a thinking something. Having reached an indubitable truth, the cogito ergo sum, Descartes moved rapidly to the assertion of the existence of God via a modified ontological argu ment. That done, God's veracity became the guarantee of the reality of the external world. Now, while everything may have seemed to be as it was before this step, the reasoning involved left Descartes with an unbridgeable gulf between the material world (res extensa ) and mind (res cogitans ). The difficulty became most acute, perhaps, when he attempted to establish the relation between soul and body, and this because of his general view of matter as a substance, a something in its own right, rather than as an element or component of substance. The unity of the human person posed the problem of making two substances, mind and body, into one substancea problem that was destined never to be solved in Cartesian terms (see mind-body problem).

The hope of Descartes was to fashion philosophy on the model of the most rigorous science, mathematics. For something to be certain, it must be clear and distinct. As the father of modern philosophy, Descartes bequeathed this ideal of rigor, the search for a method to achieve it, and an almost skeptical attitude toward sensation to a great line of followers. One of the most important thinkers in his wake was Immanuel Kant.

Kantian Dualism. Kant's critical philosophy introduced a dualism that is fundamentally epistemological.D. hume had held that experience is insufficient ground for universal and necessary judgments such as those abounding in metaphysics. Kant agreed with this and asked whether there is a source of universality and necessity elsewhere than in experience. For him, some concepts are pure and a priori in the sense that, while formed in terms of what is experienced, they are not derived from what is experienced. Such concepts as cause, effect, and substance are pure in this sense. Experience is possible because there are forms of sensuous intuition (space and time) and of understandingsubjective grooves, as it were, to which the matter of experience must conform in order to be known by man. Things as known, phenome na, are an amalgam of subjective form and objective matter; noumena are things in themselves, as such unknowable by man. This Kantian dualism has had a great impact on the philosophy of science; there Kant's theory of the forms of intuition and of understanding as what man imposes upon an amorphous and unknowable reality seems to many to explain the preponderance of theory and hypotheses in modern physical science.

Critique. Although it is possible to reduce the uses of the term "dualism" to a finite number of meanings, the term remains vague and of widely varying application. Abstractly speaking, there is little to be said for or against dualism. Whereas explanations of moral evil lead some to posit two equal principles, the one of good, the other of evil, the motive behind the reasoning is usually to deny that God is the cause of evil. The disadvantage is that God's causality seems thereby restricted and so too His preeminence. Physical dualisms, which often have epistemological sources, have the advantage of drawing a sharp distinction between the human and the nonhuman, between the spiritual and the corporeal. The disadvantage of so sharp a demarcation is that such a dualism is finally unable to account for the fact that man's vocabulary embraces the two spheres. In the final analysis, it seems that dualism is a second-order word; it is not so much a philosophical theory as a term to describe theories.

See Also: monism; persian religion, ancient; pluralism, philosophical.

Bibliography: j. hastings, ed., Encyclopedia of Religion and Ethics, 13 v. (Edinburgh 190827) 5:100114. p. foulquiÉ and r. saint-jean, Dictionnaire de la langue philosophique (Paris 1962) 190191. g. semprini, Enciclopedia filosofica, 4 v. (Venice-Rome 1957) 1:174144. j. henninger et al., Lexikon für Theologie und Kirche, ed. j. hofer and k. rahner, 10 v. (2d, new ed. Freiburg 195765) 3:582589. g. mensching and g. gloege, Die Religion in Geschichte und Gegenwart, 7 v. (3d ed. Tübingen 195765) 2:272276. r. eisler, Wörterbuch der philosophischen Begriffe, 3v. (4th ed. Berlin 192730) 1:294296.

[r. m. mcinerny]

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