Substance dualism (idea)

Substance dualism is a belief that mind and body are completely separate and different in nature, i.e. they are different substances. This view was proposed by Rene Descartes and is the impetus for the mind/body problem. It is a troublesome position because two separate substances cannot interact causally, where the mind and body clearly do interact causally. This leads to the concept of substance monism.

Descartes's Argument for Substance Dualism

Descartes begins his argument for substance dualism by establishing his existence as a certain fact, impossible to doubt. He does this through the process of discarding all that can be doubted and finding that he cannot doubt that he exists. Having established this, he goes on to investigate what this "I" who he has proven to exist is, and through the same systematic removal of all that can be doubted establishes that the essential characteristic of this "I" is that it is a thing that thinks.

So, since his essential nature is as a thinking thing, he questions whether embodiment is a necessary part of his existence. He requires that his essential nature be completely certain, and thus, since he can doubt the existence of his body and concieve of himself as a disembodied soul, his body is not part of his essential nature and therefore must be separate from his mind, whose existence is certain and impossible to doubt. Hence, mind and body are separate substances.

Rebuttal to Descartes's Argument

Descartes considers the fact that he can doubt the existence of his body as sufficient proof that it is unnecessary and thus separate. If we accept Descartes's argument as to why the body is unnecessary, it does not necessarily follow that therefore they must be separate. Having a left big toe is unnecessary in the sense that I can conceive of my left foot only having four toes, but this does not mean that it is a separate substance from the rest of my foot. Instead, it only requires the weaker claim that it is separable from the rest of the foot, i.e. that it can be made to be separate from the rest of the foot.

A Word on Substance Dualism and Identity Theory

Without this concept of substance dualism, it remains unnecessary to requirer that mental events be identical to physical events. Lacking substance dualism, the concept of the mental and the physical existing in inherently separate realms must be discarded, but it is not required to reduce mental events to physical events. The two forms of event have different characteristics. Mental and physical events retain a conceptual difference even if they are not held to completely separate domains of existence.