Most generally, the view that reality consists of two disparate parts. In philosophy of mind, the belief that the mental and physical are deeply different in kind: thus the mental is at least not identical with the physical. See occasionalism, doctrine of preestablished harmony, substance dualism, property dualism, Cartesian interactionist dualism, mind-body problem, monism.

Dualism is a time-honored philosophical position which is exemplified by:
  1. Pre-Socratics' appearance/reality distinction
  2. Plato's forms/world distinction
  3. Hume's fact/value distinction
  4. Kant's empirical phenomena/transcendental noumena distinction
  5. Heidegger's being/time distinction
  6. Russell's existence/subsistence distinction
  7. Descartes mind/matter distinction
It is, of course, the last of these which is of most immediate interest to philosophers of mind. There has been a recent revival of interest in the topic of Cartesian dualism amongst modern philosophers of mind and cognitive scientists. Arguments against dualism have been provided on the basis of both empirical evidence and on philosophical grounds, and clearly express the predominant view (Dennett, Damasio, Churchland). However, a number of modern philosophers of mind, though in the minority, have come to the defense of dualism (e.g. Hart). The question of dualism is not only of historical interest, it also has important implications for the scientific enterprise. If a convincing rejection of dualism can be formulated, the classic mind-body problem will be solved by its becoming a non-problem and the materialist approach of modern science will be vindicated (or, more accurately, some form of monism (materialism being the most plausible) will be vindicated). If, conversely, dualism can be convincingly maintained, it is by no means obvious that empirical evidence will suffice for a thorough understanding of the mind -- in other words, understanding the brain may not be enough for understanding the mind.
Descartes' mind/matter distinction can be found in his Meditations and is a particular kind of substance dualism most accurately called Cartesian interactionist dualism. Often, the term 'Cartesian dualism' is used to refer to the general class of substance dualist theories. Substance dualists hold that mind and matter are different kinds of substances. Cartesian interactionist dualism is a particular kind of substance dualism espoused by Descartes in which these two different kinds of substance can causally interact. Thus, mind substance can cause matter substance (i.e. the body) to act and matter substance (i.e. the body) can cause mind substance to have certain 'sensations' most often by itself being acted on by other material objects. For Descartes, the essence of matter is extension (i.e. having spatial dimensions and being located) whereas that of mind is active thinking. Because Descartes thought these two sorts of substance are essentially different, he held that they are also independent. Thus, matter can exist without minds and minds can exist without matter.
Descartes' position raises an important question: How do mind and matter interact? It is one thing to claim that they do interact, it is another to convincingly explain how, particularly when mind and matter are conceived of so differently. It is this question that must be answered to solve the classic mind-body problem. The Cartesian solution to the problem is to insist that the mental representation, though caused by the physical, does not resemble the physical. However, this does not seem to explain, still, how the mental comes to represent the physical at all. It seems that Descartes' final position is to insist that God is responsible for these interactions.
Solving the mind-body problem
Subsequent to Descartes, there were a number of attempts to provide dualistic solutions to the mind-body problem. These included:
Occasionalism: Espoused by Clauberg, de la Forge and Malebranche, occasionalism entails the contention that everything is devoid of causal efficacy and that God is the only truly causal agent. So, for example, placing your hand on a hot stove is does not cause pain, but is rather an occasion for God to cause the mental state of pain. So, not only mind/body interactions, but all causal interactions become the work of God.
Preestablished harmony: This doctrine was formulated by Leibniz and is basically Cartesian interactionist dualism without the interaction. Thus, rather than causal interaction, God has provided setup in which the mental and physical are synchronized so as to provide this appearance. However, it should be noted that Leibniz himself was not a dualist: for him there were no physical substances, these were just appearances. Nevertheless, this position is often considered a possible dualistic solution to the mind-body problem.
Both occasionalism and preestablished harmony are considered instances of parallelism: the view that the mental and physical realms co-occur but are not causally connected.
Modern dualists
More recently, some philosophers have suggested that the interaction problem is not due to a problem with a dualist ontology, but a problem with our notion of causation (Hart). Indeed, this problem is just as evident in physics as in dualism: a conversion of, say, light to 'psychic energy' seems no more a problem than a conversion of energy to matter. Both seem potentially able to contradict a standard notion of causation. Under this view, dualism is at least a viable possibility once we realize the difficulty may lie elsewhere than with a commitment to a dualist ontology.
Another approach to strengthening the dualist position has been to critically examine the claims that our mental lives can be adequately explained by reference to the physical brain. Philosophers of mind have, for the past ten years begun to seriously question the possibility that science will be able to close the explanatory gap between the brain and our conscious experience, or qualia (termed the 'hard problem' by Chalmers). Clearly, a reason this gap may be unbridgeable is because mind and matter are so different. Those holding this position have been called new mysterians because they insist that mind/consciousness is fundamentally mysterious and can not be explained by standard reductionist scientific means.
Chris Eliasmith