The philosophical theory of Cartesian Dualism was developed by Descartes, and the theory became so influential that the 20th Century philosopher Gilbert Ryle called it the 'official doctrine'.

Descartes' Theory

What exactly is a mind? The question is one that people have struggled with throughout recorded history. Descartes held that minds and bodies are 'substances' of distinct kinds that (in the case of humans) happen to be contingently related¹. This dualism of substances nowadays strikes most philosophers and scientists as hopelessly misguided. Until quite recently, it was widely supposed that the source of the notorious mind-body problem stemmed from the acceptance of the Cartesian picture: a solution to the problem could be achieved by rejecting dualism. However, we can begin to develop an appreciation of the mind-body problem by examining Descartes's approach to the mind.

Descartes's Distinctions between Body and Mind

Descartes' arguments trade on a logical point known as the indiscernibility of identicals (or Liebniz' law where things are 'one and the same' (numerically identical) if - and only if - what is true of one is true of the other.

The Argument From Non-spatiality of the Mind

There is a great difference between mind and body, in that body, by its nature is always divisible and that the mind is entirely indivisible ... When I consider my mind, I can distinguish no parts, but conceive myself as one single complete thing.
- Descartes

This conception of mind is often called the doctrine of 'unity of consciousness'. Descartes reasons:

• Premise one - A body is necessarily extended and so has parts.

• Premise two - A mind is not extended, and because of this it has no parts.

Therefore, minds and bodies cannot be the same kind of thing.

Material objects are spatial; they occupy a location in space and exhibit spatial dimensions. Mental objects - thoughts and sensations, however - are apparently non-spatial. What is the size and shape of your desire for a chocolate bar? Is your thinking of Manchester triangular? These questions seem to make no sense. You might think of sensations as having spatial locations; a pain in your left leg is in your left leg (but is it left leg-shaped?). Descartes considers the phenomenon of 'phantom pain'. Amputees often seem to experience pains in their amputated limbs, suggesting that although we experience pains and other sensations in various bodily locations, it need not follow that experiences of pain occur at those locations.

The Argument From First Person Authority

Descartes notices an important distinction between reports about the external world and reports about the internal world. We have authority when we report our sensations or immediate experience - the knowledge you have of your own mental states is direct and unchallengeable in a way that your knowledge of material objects is not. Think of the qualities of your experience of the pain in your leg. You may find these qualities difficult to talk about, but that need not affect your awareness of them. A neuroscientist observing your nervous system while you are experiencing pain will observe nothing resembling your pain - this possibility makes no sense.

Descartes reasons:

• Premise one - Knowledge of the external world (which includes a person's own body), is different from knowledge of the internal world.

• Premise two - The best explanation for this is that the internal and external worlds are different.

Therefore, the mind and body are distinct.

In Conclusion

Cartesian Dualism fits nicely with common sense². We see ourselves as having bodies, but being distinct from our bodies in at least the following sense; we can apparently conceive of our bodies changing dramatically, or ceasing to exist altogether, while we continue to exist. However, while you can perhaps imagine your body being destroyed while you remain, it is less clear that you could coherently imagine your surviving the demise of your mind or self³.

Related BBC Links

• Rene Descartes said 'I think, therefore I am'. Research carried out by neurologists in London now suggests that the famous dictum should be rephrased as, 'We think, therefore I am.'

• Find out which people are considered Millennium's Greatest Thinkers.