Dualism or Cartesian Dualism is the philosophical position that there are two kinds of "stuff": usual ordinary matter, and spirit. Rocks, light, clouds, etc. are composed of ordinary matter, whereas minds and souls are composed of a fundamentally different type of matter. Dualism stands in contrast to monism and philosophical materialism. Dualism is an ontological position, ontology being a branch of metaphysics.
Psychologists have found evidence that people in various cultures have a tendency to believe in mind-body dualism. This belief is a prerequisite for the idea of invisible or metaphysical minds, such as gods. Studies have found that belief in dualism is linked with religious belief.  Children seem particularly prone to dualistic thinking.
- "Children in our culture are taught that the brain is involved in thinking, but they interpret this in a narrow sense, as referring to conscious problem solving, academic rumination. They do not see the brain as the source of conscious experience; they do not identify it with their selves. [...] when asked about the psychological properties [of a fictional dead mouse], more than half the children said that these would continue: the dead mouse can feel hunger, think thoughts, and have desires. The soul survives. And children believe this more than adults do, [...] the notion that life after death is possible is not learned at all. It is a by-product of how we naturally think about the world."
Some scientists argue that belief in dualism and religious belief are evolutionary accidents.
Perhaps the major counterargument to dualism is the fact that the existence of a soul, or of soul matter, has never been demonstrated. Much experimental work has been done in psychology, but none of it has shown that there needs to be a separate type of matter to explain how minds work. As far as science can tell at present, the mind is simply the result of the operation of a brain made of ordinary matter.
The brain is resource intensive
Physiology also provides evidence against the existence of a mind outside the body: the brain is an expensive organ, requiring a lot of resources, such as oxygen, to function properly. If the seat of thought is not in the brain, but in an external mind in a separate plane, then there is no need for the human brain to be as large as it is. Natural selection should have favored those individuals with smaller brains as being better able to keep their brains functioning in times of scarcity.
Interaction not explained
If mind matter exists, it must interface with the body somehow: signals from the eyes (which are made of ordinary matter) must pass to the mind, which must then decide what to do next and somehow signal the muscles to move appropriately. It should be possible to discover where and how this mind-body interface works, for instance by studying people who have sustained mental problems as a result of physical injury or stroke.
- "What sort of mechanism could convey information of the sort bodily movement requires, between ontologically autonomous realms? To suppose that non-physical minds can move bodies is like supposing that imaginary locomotives can pull real boxcars. "
- Methodological naturalism
- Daniel Dennett presents a case against Cartesian dualism in his book, Consciousness Explained.