Mind-body dualism in the philosophy of mind maintains that the mind and body, or the mental and physical, are distinct: the mind is not the body and the body is not the mind. As a form of dualism, mind-body dualism differs from monism, the latter of which posits there to be only one fundamental property or substance. 

Mind-body dualism has been a part of philosophical discussion throughout history and hinges on what to many is an obvious observation, which is that human beings have both physical and mental properties. An individual possesses physical properties such as size, shape, weight, motion, etc. and they also seem to have mental properties like consciousness (perceptual experience, emotional experience, etc.), intentionality (beliefs, desires, etc.), and the conception of a self. Most physical properties (except for quarks and electrons, for example) are directly observable, but mental properties are private and can only be experienced directly by the subject himself. This observation has led various thinkers to posit that the mind and body are distinct.

Mind-body dualism goes back to the ancient philosopher Plato (427-347 BCE) who presented several arguments in favor of the immortality of the soul. Plato claimed that the intellect is immaterial because Forms are immaterial. The intellect must have an affinity with the Forms it apprehends. Plato maintained that this affinity is so powerful that the soul wishes to leave the physical body that imprisons it to dwell in the realm of Forms.   

More modern versions of dualism are found in French philosopher Rene Descartes’ (1596-1650) influential work Meditations on First Philosophy (1641). Descartes was a substance dualist who affirmed two kinds of substance: matter and mind. The essential property of matter, Descartes believed, is that it is spatially extended. The essential property of mind is that it thinks. Descartes articulated this in his famous dictum cogito, ergo sum (“I think, therefore I am”) which presents the mind as an immaterial, non-extended substance that engages in various activities such as rational thought, imagining, and feeling.

Mind-body dualism soon came under heavy attack. According to a mechanistic view of the world, the universe is a closed system in which everything that happens does so in accord with the laws of physics. There is no room, on such a view, for an immaterial mind to interfere with the physical world. Mechanists embraced a view of the mind called epiphenomenalism. This view holds the mind to be a by-product of the physical system that has no influence back on it. But not everyone agreed. Many philosophers continued to see the mind or mental states as somehow distinct from the physical body. These thinkers viewed mental states as imperative for understanding human responses to the world. 

The result has been a small revival of dualism in the last decade of the twentieth century and several important questions continue to be debated today. There is the question of ontology that attempts to answer the question: what are mental states and physical states? Are all mental states the same as physical states, or are they entirely distinct? Another question concerns causality: do physical states influence mental states? Do mental states influence physical states? If they do, then how? There is also the problem of consciousness: what is consciousness and how is it related to the brain and the body?

Dualism is not restricted to the substance dualism that we saw was held by Descartes. Property dualism is another form that affirms only one fundamental material substance that has two different kinds of property: physical properties and mental properties. On such a view, the brain has physical properties (like mass and width) and mental properties (like a thought, feeling, or idea). Both types of property are different but there is only one substance. Property dualists view the mind as still connected to the body. The mind (consciousness) will end with the death of the body. Property dualism is an alternative to physicalist views of the mind that maintain that all properties in the universe can be reduced to physical properties.