Substance Dualism

Cartesian interactionist substance dualism: explanation, arguments for, arguments against.

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Descartes and Substance Dualism

Descartes claimed that there are two types of substances: mental substance and physical substance. the body and mind are separate - your body is extended in space (principle attribute = extension) but your mind is not (principle attribute = thought). Descartes tries to explain that the mind and body are two wholly different substances:


  • I can conceive of myself as having no body
  • from the fact that I am thinking, I cannot conceive that "I" (as a thinking thing) do not exist - cogito ergo sum
  • what I cannot doubt cannot be the same as what I can doubt
  • therefore, "I", as a thinking thing, am not identical with my body


  • Body, by definition, has the principle attribute of extension, and what is extended in space is divisible
  • Mind, in contrast, is non-extended and by definition indivisible ("when I consider my mind, that is to say myself insofar as I am only a thinking thing, I can distinguish no parts")
  • Therefore, "I", a thinking thing, am not identical with my body
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Substance Dualism Evaluated

Problem with indubitibility argument:

Arnauld Just because one can doubt the existence of something does not necessarily make it a plausible claim. For example I can conceive of of a right-angled triangle lacking the Pythagorean property but it does not follow that the triangle lacks it, or that it is not an essential property to the triangle. 

Problem with indivisibility argument:

According to Hume, the mind is divisible because it is in fact just a bundle of perceptions: the "I" is just a convenient linguistic convention. Even if it is possible that consciousness is single and indivisible it does not follow that there isn't something physical that does the thinking and which IS divisible. 


  • I have the desire (mental state) to eat a cookie and at the same time that I have this desire my brain goes through a certain process (physical state). 
  • Then my brain goes through a certain process that causes my hand to place the cookie in my mouth (physical state). 
  • My hand then raises and I eat the cookie (outward physical behaviour


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Interactionist Property Dualism

This view holds that mental states can and often do interact causally with physical states. This is the common sense view that most non-philosophers hold e.g. if a squirrel bites my ear then I feel pain - a physical state has caused a mental state.


Causal interaction is inconceivable if two things are not ontologically similar enough i.e. how can the material and immaterial impact upon each other - they simply lack that communality necessary for interaction


Causal closure of the physical universe: the material world is causally closed i.e. immaterial minds cannot affect the material world. Physical energy cannot simply appear out of thin air (which is implied by substance dualists when a 'mental state causes a physical state'.

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