In the philosophy of mind, dualism is a set of views about the relationship between mind and matter, which begins with the claim that mental phenomena are, in some respects, nonphysical.

The first manifestations of mind/body dualism probably go back into the mists of time, from the point where man first began to speculate about the existence of an incorporeal soul (or spirit) which bore the faculties of intelligence and wisdom. We first encounter similar ideas in Western philosophy with the writings of Plato and Aristotle, who maintained, for different reasons, that man's "intelligence" (a faculty of the mind or soul) could not be identified with, or explained in terms of, his physical body.

However, the best-known version of dualism is due to René Descartes (1641), and holds that the mind is a nonphysical substance. Descartes was the first to clearly identify the mind with consciousness and self-awareness and to distinguish this from the brain, which was the seat of intelligence. Hence, he was the first to formulate the mind/body problem in the form in which it still exists today. These days, dualism is contrasted with various kinds of monism, including physicalism and phenomenalism. Substance dualism is contrasted with all forms of materialism, but property dualism may be considered a form of emergent materialism and thus would only be contrasted with non-emergent materialism. This article discusses the various forms of dualism and the arguments which have been made both for and against this thesis.

Historical overview

Plato and Aristotle

In the dialogue Phaedo, Plato formulated his famous doctrine of eternal Forms as distinct and immaterial substances of which the objects and other phenomena that we perceive in the world are nothing more than mere shadows. Plato's doctrine was the prototypical conception of all future manifestations of, what is now called, substance dualism in ontology. But Plato's doctrine of the Forms is not to be considered as some sort of ancient and superseded metaphysical notion because it has precise and undeniable implications for the philosophy of mind and the mind-body problem in particular.

Plato makes it clear, in the Phaedo, that the Forms are the universalia ante rem, i.e. they are universal concepts (or ideas) which make all of the phenomenal world intelligible. Consequently, in order for the intellect (the most important aspect of the mind in philosophy up until Descartes) to have access to any kind of knowledge with regard to any aspect of the universe, it must necessarily be a non-physical, immaterial entity (or property of some such entity) itself. So, it is clear on the basis of the texts that Plato was a very powerful precursor of Descartes and his subsequent more stringent formulation of the doctrine of substance dualism.

Aristotle strongly rejected Plato's notion of Forms as independently existing entities. In the Metaphysics, he already points to the central problems with this idea. On the one hand, if we say that the particulars of the phenomenal world participate or share in the Form, we seem to be destroying the Form's essential and indispensable unity. On the other, if we say that the particulars merely resemble, or are copies of, the Form, we seem to need an extra form to explain the connection between the members of the class consisting of the-particulars-and-the-form, and so on, leading to an infinite regress. This is what Aristotle referred to as the problem of the third man.

For these reasons, Aristotle revised the theory of forms so as to eliminate the idea of their independent existence from concrete, particular entities. The form of something, for Aristotle, is the nature or essence (ousia, in Greek) of that thing. To say that Socrates and Callias are both men is not to say that there is some transcendent entity "man" to which both Socrates and Callias belong. The form is indeed substance but it is not substance over and above the substance of the concrete entities which it characterizes. Aristotle rejects both univeralia in rebus as well as universalia ante rem. Some philosophers and thinkers have taken this to be a form of materialism and there may be something to there arguments. However, what is important from the perspective of philosophy of mind is that Aristotle does not believe that intellect can be conceived of as something material. He argues as follows: if the intellect were material then it could not receive all of the forms. If the intellect were a specific material organ (or part of one) than it would be restricted to receiving only certain kinds of information, as the eye is restricted to receiving visual data and the ear is restricted to receiving auditory data. Since the intellect is capable of receiving and reflecting on all forms of data, then it must not be a physical organ and, hence, it must be immaterial.

From Neoplatonism to Scholasticism

Early Christianity seems to have struggled to come to terms with the identification of a unique position with regard to the question of the relationship between mind and body, just as it struggled to define the relationship of the ontological status of Christ himself (see homoousianism, homoiousianism, Arianism, etc). A consensus seemed to emerge around, what is now called, Neoplatonism in the early Middle Ages. The doctrines of Neoplatonism were essentially minor modifications on Plato's general ideas about the immortality of the soul and the nature of the Forms. The Neoplatonic Christians identified the Forms with souls and believed that the soul was the substance of each individual human being, while the body was just a shadow or copy of these eternal phenomena.

Later philosophers, following in the neo-Aristotelian trail blazed by Thomas Aquinas, came to develop a trinitarian notion of forms which paralleled the Trinitarian doctrine of Father, Son and Holy Spirit: forms, intellect and soul were three aspects or parts of the same singular phenomenon. For Aquinas, the soul (or intellect) remained the substance of the human being, but, somewhat similarly to Aristotle's proposal, it was only through its manifestation inside the human body that a person could be said to be a person. While the soul (intellect or form) could exist independently of the body (unlike in Aristotle)the soul by itself did not constitute a person. Hence, Aquinas suggested that instead of saying "St. Peter pray for us" one should rather say something like "soul of St. Peter pray for us", since all that remained of St. Peter, after his death, was his soul.All things connected with the body, such as personal memories, were cancelled out with the end of one's corporeal existence.

There are different views on this question in modern Christianity. Official Catholic Church doctrine, as illustrated by the Apostle's Creed, claims that the body (presumably transformed into an immaterial entity) is resurrected at death and, along with the soul, goes to Heaven or Hell after death. Hence, there is a sort of inseparability of soul, mind and body which is even more strongly reminiscent of Aristotle than the positions expressed by Thomas Aquinas.

Many theologians do not accept this doctrine and insist, instead, that only the immaterial soul (and hence mind or intellect) goes to Heaven, leaving the body (and brain) behind it forever.

Descartes and his disciples

In his Meditations on First Philosophy, Descartes embarked upon a quest in which he called all his previous beliefs into doubt, in order to find out what he could be certain of. In doing so, he discovered that he could doubt whether he had a body (it could be that he was dreaming of it or that it was an illusion created by an evil demon), but he couldn't doubt whether he had a mind. This gave Descartes his first inkling that the mind and body were different things. The mind, according to Descartes, was a "thinking thing" (lat. res cogitans), and an immaterial substance. This "thing" was the essence of himself, that which doubts, believes, hopes, and thinks. The distinction between mind and body is argued in Meditation VI as follows: I have a clear and distinct idea of myself as a thinking non-extended thing, and a clear and distinct idea of body as an extended and non-thinking thing. Whatever I can conceive clearly and distinctly, God can so create. So, Descartes argues, the mind, a thinking thing, can exist apart from its extended body. And therefore, the mind is a substance distinct from the body, a substance whose essence is thought.

The central claim of Cartesian dualism is that the immaterial mind and the material body causally interact, an idea which continues to feature prominently in many non-European philosophies. Mental events cause physical events, and vice-versa. This leads to a very substantial problem

Descartes himself struggled to come up with a feasible answer to this problem. In his letter to Queen Elizabeth I, he suggested that animal spirits interacted with the body through the pineal gland, a small gland in the centre of the brain, between the two hemispheres. However, this explanation was not satisfactory: how can an immaterial mind interact with the physical pineal gland? Because Descartes' was such a difficult theory to defend, some of his disciples, such as Arnold Geulincx and Nicholas Malebranche, proposed a different explanation: That all mind-body interactions required the direct intervention of God. According to these philosophers, the appropriate states of mind and body were only the occasions for such intervention, not real causes. These occasionalists maintained the strong thesis that all causation was directly dependent on God, instead of holding that all causation was natural except for that between mind and body.

Types of ontological dualism

Ontological dualism makes dual commitments about the nature of existence as it relates to mind and matter, and can be divided into three different types:

(1) Substance dualism asserts that mind and matter are fundamentally distinct kinds of substances.

(2) property dualism suggests that the ontological distinction lies in the differences between properties of mind and matter (as in emergentism).

(3) predicate dualism claims the irreducibility of mental predicates to physical predicates.

Substance dualism

Substance dualism is a type of dualism most famously defended by Descartes, which states that there are two fundamental kinds of substance: mental and material. The mental does not have extension in space, and the material cannot think. Substance dualism is a viewpoint which contradicts physicalism, one of the most popular views in modern philosophy of mind. However, it may be regarded as important historically in that it has given rise to much thought over the famous mind-body problem. It may also be noted that philosophical interpretations of quantum mechanics -- especially the consciousness causes collapse interpretation -- are not a revival of substance dualism, since these views generally claim the observer is entangled in the object being observed, not a separate substance as in the case of substance dualism. Substance dualism is a philosophical position compatible with most theologies which claim that immortal souls occupy an independent "realm" of existence which is distinct from that of the physical world.

David Chalmers recently developed a thought experiment inspired by the movie The Matrix in which substance dualism could be true: Consider a computer simulation in which the bodies of the creatures are controlled by their minds and the minds remain strictly external to the simulation. The creatures can do all the science they want in the world, but they will never be able to figure out where their minds are, for they do not exist in their observable universe. This is a case of substance dualism with respect to computer simulation. This naturally differs from a computer simulation in which the minds are part of the simulation. In such a case, substance monism would be true.

Property dualism

Property dualism asserts that when matter is organized in the appropriate way (i.e. in the way that living human bodies are organized), mental properties emerge. Hence, it is a sub-branch of emergent materialism. Different versions of property dualism describe this in different ways. Epiphenomenalism asserts that while material causes give rise to sensations, volitions, ideas, etc., such mental phenomena themselves cause nothing further: they are causal dead-ends. Interactionism, on the other hand, allows that mental causes can produce material effects, and vice-versa.

Biological naturalism

John Searle's biological naturalism is sometimes mistakenly viewed as a kind of property dualism, since, in Searle's view, a person's mental properties are categorically different from his or her micro-physical properties. The latter have "third-person ontology" whereas the former have "first-person ontology." Micro-structure is accessible objectively by any number of people, as when several brain surgeons inspect a patient's cerebral hemispheres. But pain or desire or belief are accessible subjectively by the person who has the pain or desire or belief, and no one else has that mode of access. However, Searle understands mental properties to be a species of physical property -- ones with first-person ontology. So this sets his view apart from a dualism of physical and non-physical properties. His mental properties are putatively physical.

Searle believes that consciousness "is a real part of the real world and it cannot be eliminated in favor of, or reduced to, something else" whether that something-else is a neurological state of the brain or a software program. He contends, for example, that the software known as Deep Blue knows nothing about chess. He also believes that consciousness is both a cause of events in the body and a response to events in the body.

On the other hand, Searle doesn't treat consciousness as a ghost in the machine. He treats it, rather, as an emergent property of the brain as a whole. (See holism). The causal interaction of mind and brain can be described thus in naturalistic terms: Events at the micro-level (perhaps at that of individual neurons) cause consciousness. Changes at the macro-level (the whole brain) constitute consciousness. Micro-changes cause and then are impacted by holistic changes, in much the same way that individual football players cause a team (as a whole) to win games, then the individuals gain confidence from the knowledge that they are part of a winning team.

Predicate dualism

Predicate dualism is the view espoused by most non-reductive physicalists, such as Donald Davidson and Jerry Fodor, who maintain that while there is only one ontological category of substances and properties of substances (usually physical), the predicates that we use to describe mental events cannot be redescribed in terms of (or reduced to) physical predicates of natural languages. If we characterize predicate monism as the view subscribed to by eliminative materialists, who maintain that such intentional predicates as believe, desire, think, feel, etc., will eventually be eliminated from both the language of science and from ordinary language because the entities to which they refer do not exist, then predicate dualism is most easily defined as the negation of this position. Predicate dualists believe that so-called "folk psychology", with all of its propositional attitude ascriptions, is an ineliminable part of the enterprise of describing, explaining and understanding human mental states and behavior.

Davidson, for example, subscribes to Anomalous Monism, according to which there can be no strict psycho-physical laws which connect mental and physical events under their descriptions as mental and physical events. However, all mental events also have physical descriptions. It is in terms of the latter that such events can be connected in law-like relations with other physical events. Mental predicates are irreducibly different in character (rational, holistic and necessary) from physical predicates (contingent, atomic and causal).

Types of interaction dualism

Interactionism

Interactionism is the the view that mental states, such as beliefs and desires, causally interact with physical states. This is a position which is very appealing to common-sense intuitions, notwithstanding the fact that it is very difficult to establish its validity or correctness by way of logical argumentation or empirical proof. It is appealing to common-sense because we are surrounded by such everyday occurrences as a child's touching a hot stove (physical event) which causes him to feel pain (mental event) and then yell and scream (physical event) which causes his parents to experience a sensation of fear and protectiveness (mental event) and so on.

One of the main objections to dualistic interactionism is that it is very difficult, if not impossible, to understand, how two completely different types of substances (material and immaterial) are able to interact causally. One response to this is to point out that, perhaps, the causal interaction that takes place is not of the classical "billiard ball" type of Newtonian mechanics but instead involves energy, dark matter or some other such mysterious processes.

Even if the latter is true, it has been argued though, there is still a problem: such interactions seem to violate the fundamental laws of physics. If some external and unknown source of energy is responsible for the interactions, for example, then this would violate the law of the conservation of energy. On the other hand, the conservation laws only apply to closed and isolated systems and since human beings are not closed and isolated systems, an interactionist would argue, then the laws of conservation absolutely do not apply in this case.

Another argument against interactionism is that it violates a general heuristic principle of science: the causal closure of the physical world. But Mills has responded to this by pointing out that mental events may be causally overdetermined. Causal overdetermination means that some features of an effect may not be fully explained by its sufficient cause. For example, "the high pitched music caused the glass to break but this is the third time that that glass has broken in the last week." It is certain that the high-pitched music is the sufficient cause of the breaking of the glass, but it does not explain the feature of the event that is identified by the phrase "this is the third time this week..". That feature is causally related, in some sense, to the two prior events of the glasses having broken in the last week. In response, it

Moreover, there is the question of determinism versus indeterminism. In quantum mechanisms, events at the microscopic level (at least) are indeterminate. The more precisely I can localize the position of an electron, the more imprecise becomes my ability to determine its angular momentum and vice-versa. Philosophers such as Karl Popper and John Eccles have theorized that such indeterminacy may apply even at the macroscopic scale. Most scientists insist that the effects of such indeterminacy cancel each other out at larger levels.

Epiphenomalism