Arguments for Dualism
Descartes presented a series of arguments purporting to prove that there is a real distinction between mind and body. Each of these arguments for substance dualism has the same logical structure, first identifying a property that the body has but which the mind seems to lack, and inferring that because they have different properties they cannot be identical.
Arguments such as these rest on a principle associated with Leibniz: the principle of the indiscernability of identicals. This principle states that if two things are one and the same (identical) then it is impossible to tell them apart (they are indiscernable).
Descartes’ arguments attack the monist view that a person is identical with their body, that a person and his body are exactly the same thing. If this monist view is true, Descartes suggests, then whatever is true of the body will also be true of the person.
There are some things, though, Descartes observes, that are true of my body that are not true of me: the argument from feigning rests on the idea that while we can feign that our bodies do not exist, we cannot feign that our minds do not exist; the argument from divisibility rests on the idea that while our bodies are divisible we ourselves are not; the argument from extension rests on the idea that while our bodies have a spatial location, our minds do not.
Each of these arguments infers that mind and body are discernable, that we can tell them apart. There are some things that are true of my body that are not true of me. I therefore cannot be exactly the same thing as my body; I must be something more.