Cartesian, or substance, dualism

  • One traditional definition of a substance is something that does not depend on any other thing to exist.
  • Substance dualism holds that there are two types of substances, mental substances (minds) and material substances (bodies), each capable of existing without the other.
  • Materialism holds that there are only material substances, so everything that exists either is, or depends on, a material substance to exist.
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Reasons for holding this view

  • Plato argues that, unlike the body, the soul cannot be destroyed because it does not have parts. We can reply that there can be other kinds of destruction.
  • He also argues that all change involves something coming about from its opposite. Becoming alive is a change from not being alive. It is the joining of the soul to the body; so the soul must exist before life. We can reply that 'coming into existence' doesn't involve one's changing from one thing to another. 
  • Descartes argues that he can doubt the existence of his body, but not the existence of his mind, which shows that his mind can exist without his body.
  • We can object that he has not shown he/his mind exists as a substance, a unitary thing. He could be no more than a succession of thoughts.
  • Another objection is that we cannot legitimately infer from conceiving of the mind existing without the body that it can actually exist without the body. Being able to conceive of two things as different does not guarantee that they are two different things, rather than one thing thought of in two different ways.
  • Descartes argues that having parts is an essential property of bodies, as things that exist in space. But the essential property of minds is thought; and minds have no parts. Therefore, minds and bodies are distinct.
  • We can object that this presupposes that minds exist (so as to have properties).
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Problems associated with this view of mind

  • Substance dualism entails that we are two things, mind and body, connected together. We can object that our experience is of being just one thing.
  • Neuroscience has shown that the mind is very dependent on the brain, which undermines the idea that the mind is a separate substance. Descartes can respond that the dependence is causal, not logical. 
  • Substance dualism faces the problem of explaining how the mind, given that it is so different from the body, can cause physical events.
  • Descartes assumes we can make sense of the idea of our minds existing alone, without any other mind or physical world. This entails that words must get their meaning by referring to our ideas, thoughts and sensations. But if this is true, the word 'experience' means my experience. Since it is logically impossible that anyone else should have my experience it is logically impossible that anyone else should have experience. This is solipsism.
  • Substance dualism also faces the challenge of showing that we can know other minds exist. If minds are logically independent of bodies, any evidence from someone's bodily behaviour does not prove that they have a mind.
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Responses to these problems

  • The argument from analogy claims that I can infer other people have minds because they behave as I do, and I have a mind.
  • We can object that we cannot base an inference on one case. I could be a special case.
  • The argument from inference to the best explanation claims that the hypothesis that other people have minds is the best explanation for their behaviour. We can develop this by saying that mental states are defined by their causal relations.
  • We can object, first, that this account of mental states is false, and second, that it presupposes that we can show that mental states cause behaviour. We can also object that the belief that other people have minds is not a hypothesis at all.
  • Wittgenstein argued that we can see mental states expressed in behaviour, for example facial expression, and that this is not an inference.
  • We can also argue that the ability to ascribe mental states to oneself presupposes that one can ascribe them to other people. To become a mind presupposes the existence of other minds. This also provides an answer to solipsism. 
  • The private language argument says that words cannot get their meaning by referring to 'private' sensations or ideas, because this provides no criterion for using the word correctly. That words can be used correctly or incorrectly presupposes a public standard of meaning.
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