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Introduction Back to Top

Dualism in Metaphysics is the belief that there are two kinds of reality: material (physical) and immaterial (spiritual). In Philosophy of Mind, Dualism is the position that mind and body are in some categorical way separate from each other, and that mental phenomena are, in some respects, non-physical in nature.

It can be contrasted (both as a metaphysical concept and as regards Philosophy of Mind) with various kinds of Monism (including Physicalism and Idealism), and with Pluralism, which holds that ultimately there are many kinds of substance, rather than just two.

Dualism appeals to the common-sense intuition of the vast majority of non-philosophically-trained people, and the mental and the physical do seem to most people to have quite different, and perhaps irreconcilable, properties. Mental events have a certain subjective quality to them (known as qualia or "the ways things seem to us"), whereas physical events do not. Critics of dualism have often asked how something totally immaterial can affect something totally material (the problem of causal interaction). With the knowledge gained from modern science, few, if any, neuroscientists would consider taking a dualist position, and Monistic beliefs like Physicalism are now much more common within the field of philosophy.

History of Dualism Back to Top

Dualism can be traced back to Plato and Aristotle, and also to the early Sankhya and Yoga schools of Hindu philosophy.

Plato first formulated his famous Theory of Forms, distinct and immaterial substances of which the objects and other phenomena that we perceive in the world are nothing more than mere shadows. He argued that for the intellect to have access to these universal concepts or ideas, the mind must itself be a non-physical, immaterial entity.

Aristotle argued that if the intellect were a specific material organ (or part of one) then it would be restricted to receiving only certain kinds of information (in the same way as the eye is restricted to receiving visual data). Since the intellect is capable of receiving and reflecting on all forms of data, then it must not be a physical organ and so must be immaterial.

Neo-Platonic Christians identified Plato's 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. For St. Thomas Aquinas, the soul was still the substance of the human being but, similar to Aristotle's proposal, it was only through its manifestation inside the human body that a person could be said to be a person.

However, Dualism was most precisely formulated by René Descartes in the 17th Century. Descartes was the first to formulate the mind-body problem in the form in which it exists today, and the first to clearly identify the mind with consciousness and self-awareness, and to distinguish this from the brain, which was the physical seat of intelligence. He realized 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 could not doubt whether he had a mind, which suggested to him that the mind and body must be different things. However, the immaterial mind and the material body, while being ontologically distinct substances, causally interact in some unspecified way through the pineal gland.

Types of Dualism Back to Top
  • Substance Dualism (or Cartesian Dualism) argues that the mind is an independently existing substance - the mental does not have extension in space, and the material cannot think. This is the type of Dualism most famously defended by Descartes, and it is compatible with most theologies which claim that immortal souls occupy an independent "realm" of existence distinct from that of the physical world.
  • Property Dualism (also sometimes known as Token Physicalism) maintains that the mind is a group of independent properties that emerge from the brain, but that it is not a distinct substance. Thus, when matter is organized in the appropriate way (i.e. in the way that living human bodies are organized), mental properties emerge.
    There are three main types of Property Dualism:
    • Interactionism, which allows that mental causes (such as beliefs and desires) can produce material effects, and vice-versa. Descartes believed that this interaction physically occurred in the pineal gland.
    • Occasionalism, asserts that a material basis of interaction between the material and immaterial is impossible, and that the interactions were really caused by the intervention of God on each individual occasion. Nicholas Malebranche was the major proponent of this view.
    • Parallelism (or Psychophysical Parallelism), holds that mental causes only have mental effects, and physical causes only have physical effects, but that God has created a pre-established harmony so that it seems as if physical and mental events (which are really monads, completely independent of each other) cause, and are caused by, one another. This unusual view was most prominently advocated by Gottfried Leibniz.
    • Epiphenomenalism, which asserts that mental events are causally inert (i.e. have no physical consequences). Physical events can cause other physical events, and physical events can cause mental events, but mental events cannot cause anything, since they are just causally inert by-products of physical events which occur in the brain (i.e. epiphenomena) of the physical world. This doctrine was first formulated by Thomas Henry Huxley in the 19th Century, although based on Thomas Hobbes' much earlier Materialism theories.
  • Predicate Dualism argues that more than one predicate (how we describe the subject of a proposition) is required to make sense of the world, and that the psychological experiences we go through cannot be redescribed in terms of (or reduced to) physical predicates of natural languages.
  • Epistemological Dualism (also known as Representationalism or Indirect Realism) is the view in Epistemology that the world we see in conscious experience is not the real world itself, but merely a miniature virtual-reality replica of that world in an internal representation.
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