Dualism begins with the claim that mental phenomena are, in some respects, non-physical. One of the earliest known formulations of mind-body dualism existed in the eastern Sankhya and Yoga schools of Hindu philosophy (c. 650 BCE) which divided the world into purusha (mind/spirit) and prakti (material substance). Specifically, the Yoga Sutra of Patanjali presents an analytical approach to the nature of mind. In the Western philosophical tradition, we first encounter similar ideas 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 Descartes, and holds that the mind is a non-physical 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.
The main argument in favour of dualism is simply that it appeals to the common-sense intuition of the vast majority of non-philosophically-trained people. If asked what the mind is, the average person will usually respond by identifying it with their self, their personality, their soul, or some other such entity, and they will almost certainly deny that the mind simply is the brain or vice-versa, finding the idea that there is just one ontological entity at play to be too mechanistic or simply unintelligible. The majority of modern philosophers of mind reject dualism, suggesting that these intuitions, like many others, are probably misleading. We should use our critical faculties, as well as empirical evidence from the sciences, to examine these assumptions and determine if there is any real basis to them.
Another very important, more modern, argument in favor of dualism is the idea that the mental and the physical seem to have quite different and perhaps irreconcilable properties. Mental events have a certain subjective quality to them, whereas physical events obviously do not. For example, what does a burned finger feel like? What does blue sky look like? What does nice music sound like? Philosophers of mind call the subjective aspects of mental events qualia (or raw feels). There is something that it is like to feel pain, to see a familiar shade of blue, and so on; there are qualia involved in these mental events. And the claim is that qualia seem particularly difficult to reduce to anything physical.
Interactionist dualism, or simply interactionism, is the particular form of dualism first espoused by Descartes in the Meditations. In the 20th century, its major defenders have been Popper and John Carew Eccles. It is the view that mental states, such as beliefs and desires, causally interact with physical states. Descartes’ famous argument for this position can be summarized as follows: Seth has a clear and distinct idea of his mind as a thinking thing which has no spatial extension (i.e., it cannot be measured in terms of length, weight, height, and so on) and he also has a clear and distinct idea of his body as something that is spatially extended, subject to quantification and not able to think. It follows that mind and body are not identical because they have radically different properties, according to Descartes.
At the same time, however, it is clear that Seth’s mental states (desires, beliefs, etc.) have causal effects on his body and vice-versa: a child touches a hot stove (physical event) which causes pain (mental event) and makes him yell (physical event) which provokes a sense of fear and protectiveness in the mother (mental event) and so on.
Descartes’ argument obviously depends on the crucial premise that what Seth believes to be “clear and distinct” ideas in his mind are necessarily true. Most modern philosophers doubt the validity of such an assumption, since it has been shown in modern times by Freud (a third-person psychologically-trained observer can understand a person’s unconscious motivations better than he does), by Duhem (a third-person philosopher of science can know a person’s methods of discovery better than he does), by Malinowski (an anthropologist can know a person’s customs and habits better than he does), and by theorists of perception (experiments can make one see things that are not there and scientists can describe a person’s perceptions better than he can), that such an idea of privileged and perfect access to one’s own ideas is dubious at best.
Other important forms of dualism which arose as reactions to, or attempts to salvage, the Cartesian version are:
1) Psycho-physical parallelism, or simply parallelism, is the view that mind and body, while having distinct ontological statuses, do not causally influence one another, but run along parallel paths (mind events causally interact with mind events and brain events causally interact with brain events) and only seem to influence each other. This view was most prominently defended by Leibniz. Although Leibniz was actually an ontological monist who believed that only one fundamental substance, monads, exists in the universe and everything else is reducible to it, he nonetheless maintained that there was an important distinction between “the mental” and “the physical” in terms of causation. He held that God had arranged things in advance so that minds and bodies would be in harmony with each other. This is known as the doctrine of pre-established harmony.
2) Occidentalism is the view espoused by Nochilas Malebranche which asserts that all supposedly causal relations between physical events or between physical and mental events are not really causal at all. While body and mind are still different substances on this view, causes (whether mental or physical) are related to their effects by an act of God’s intervention on each specific occasion.
3) Epiphenomenalism is a doctrine first formulated by T.H. Huxley. Fundamentally, it consists in the view that mental phenomena are causally inefficacious. 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 (i.e. epiphenomena) of the physical world. The view has been defended most strongly in recent times by Frank Jackson.
4) 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. These emergent properties have an independent ontological status and cannot be reduced to, or explained in terms of, the physical substrate from which they emerge. This position is espoused by David Chalmers and has undergone something of a renaissance in recent years.