René Descartes

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Explain the theory of substance (Cartesian) dualism. Consider the nature of minds and bodies, according to Descartes.

According to the Cartesian theory of substance dualism, there are two substances, mind and body. For Descartes, thought constitutes the nature of the mind, whereas extension constitutes the nature of the body.

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Descartes defines substance as anything that doesn't depend on anything else for its existence. In other words, a substance is that which is self-subsistent. Strictly speaking, this quality is only possessed by God, as only God is self-subsistent, whereas everything else depends on God for its existence.

Nevertheless, Descartes departs from his initial definition and broadens his concept of substance by extending it to mind and body. As Descartes mainly deals with two substances and their interaction, his theory has been described as substance dualism. The two substances in question are mind and body.

Each substance has its own unique nature, or essence. The nature of mind is thought, and the nature of body is extension. Another way of putting this is to say that a mind is a substance that thinks, whereas a body is a substance that is extended. In philosophy, to be extended simply means to take up space in the physical world. Human bodies are extended, as indeed are rocks, plants, trees, and animals.

As Descartes argues that mind and body are two wholly distinct substances with completely different essences or natures, he has to deal with the problem of how they can possibly interact. It would seem that, on a basic level, there does indeed appear to be some kind of interaction between mind and body, such as when I think about doing something and then act on my thoughts.

But this is not enough for Descartes. He needs to be able to offer a fully worked out explanation of the relationship between mind and body if his metaphysical system is going to have the comprehensiveness he seeks.

However, anyone looking for a plausible account of mind-body interaction in Descartes is likely to be disappointed. Descartes conjectured, somewhat fancifully, that the brain's pineal gland provided some kind of bridge between mind and body. Later philosophers countered Descartes by arguing that causation only has meaning between objects in the physical world—in other words, extended objects. As minds are not objects in the physical world, so the argument runs, there could not be any causal interaction between them and bodies.

Descartes was certain that there was some kind of interaction between the two substances, but his inability to give a credible, adequate account of how this happened was undoubtedly one of the main weaknesses of his philosophical system.

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