Marie asked:

My question is about Descartes’ dualism, MindBody Problem, I don’t quite understand it; I don’t get how he came to the conclusion that mind and body are two different substance. I’m hoping that you would be able to explain it to me please. Thank you!

Answer by Jürgen Lawrenz

it is very simple. Just look at the world and ask yourself: Is everything material? Or are there things in the world that are not material, but I still know they exist?

Then, ask yourself a second question: how can I arrive at a definition so that the two cannot be confused?

So now: Descartes said that every material thing is defined by having extension. Which is another way of saying: it occupies space. Moreover it cannot share that space with another things. Even water or gas can be reduced to particles, and then you find they are extended things. So each occupies a unique portion of space.

So the first substance, which he called ‘res extensa’, is clearly matter. What about my mind, my emotions and beliefs? Well, they are not things in that sense. They are not extended, so do they do not occupy space. So they must be a non-material substance. These he called ‘res cogitans’, or thought-like things.

And now the confusion between them stops. If you can measure something in space, as having a length, breadth, width etc., then it must be a material substance. If you can’t measure it, then it must be a thought-like substance.

This is the origin of his dual-substance doctrine. A human being is both matter (flesh and bones) and thought-things (mind, emotions etc.).

One little addition: In Descartes’ time people believed in ghosts. But after Descartes it was impossible to believe in them any more. Since you can’t see thought-like things, not even under a microscope, how come we can see ghosts? So there is something fishy! If you can see them, they must be material things! But now you can’t catch a ghost, and immediately you have a self-contradiction on your hand. Conclusion: There are no ghosts, only people believing they saw them, in other words, hallucinating.

Hope this helps!

Answer by Oliver Leech

Below are the arguments presented by Descartes himself to justify his substance dualism.

(There are many other arguments that could be used in support of his position and many objections to substance dualism. )


Descartes proves his own existence as a mind or thinking thing (res cogitans) in Meditation II. He proves the existence of his body in his wider proof of material things by the end of Meditation VI. The essential principle of Cartesian dualism is that mind and body are not identical but quite distinct, separate substances.

He gives three proofs that the mind and the body are distinct. An important implication of this argument is that the mind, as a separate substance, might exist without the body.

Leibniz’s Law: for two objects to be one and the same thing, whatever is true of the one must be also true of the other. If Descartes can show that the mind has different properties from the body, then, by Leibniz’s Law, he has shown that the mind and the brain are not identical:


a. Reference: Discourse Part 4 and Meditation II.


i. I can doubt that I have a body.

ii. I cannot doubt that I am.

iii. Therefore, I who am doubting and thinking am not a body.

c. In short, I cannot doubt that I exist (as a mind, a thinking thing, res cogitans) but I can doubt that I have a body (a physical object). Therefore, I (the mind, the thinking thing) am not the same substance as my body.

d. Present the argument as a formula:

i. I cannot doubt the existence of M.

ii. I can doubt the existence of B.

iii. Therefore, M is not the same as B.

e. Present analogous arguments, using the same formula.

i. Represent mashed potato as M and carbohydrate as B.

ii. I cannot doubt the existence of M.

iii. I can doubt the existence of B.

iv. Therefore, mashed potato is not the same as carbohydrate (and, by implication, mashed potato could exist without carbohydrate).

v. Clearly there is something wrong with this argument.

f. Another analogy: Lois Lane thinks:

i. I do not doubt that Clark Kent is a journalist.

ii. I doubt that Superman is a journalist.

iii. Therefore, Clark Kent is not Superman.

iv. (But Clark Kent is Superman. There must be something wrong with the argument. )

g. Try Dr Jekyll and Mr Hyde or devise your own example to make the point.

h. Perhaps the fallacy in the argument is that when Descartes introduces the ideas of doubting and of not doubting, he is referring to two different ways of thinking that he finds in himself. Doubting and not doubting are not attributes of the mind and of the body but of his response to the mind and the body. Consequentially, the fact that they are different tells us nothing about the mind and the body, whether they are or they are not different. If Descartes had argued that the mind was indubitable (incapable of being doubted) and that the body was dubitable (capable of being doubted), then the mind and the body would have different properties and, therefore, by Leibniz’s law be different things. But Descartes has not shown that this is the case, only that he can doubt one and not doubt the other.


a. Reference: Meditation VI: ‘there is a great difference between mind and body, in that body, by its nature, is always divisible and that mind is entirely indivisible… my mind… a thinking thing [has] no parts… but [is] one single and complete thing.’

Read through the argument.

b. DIVISIBILITY: the body, a physical thing, can be described in terms of the quantitative language of physics, that is, it has size, shape, extension, motion; as such it is divisible. For example, it is easy to conceive that anything that has dimensions might be divided into smaller parts. Any number can be divided and so any measurement can be divided.

c. INDIVISIBILITY: the mind, by contrast, is thought of in qualitative terms. Particular states of mind, for example, the sound of a violin, the smell of perfume, seeing the blue of a clear sky seem not capable of being divided. How could you have half a sound or smell? (You might have a quieter sound, i.e. , half the volume or a less intense smell but that is not the same as dividing. )

d. In short, we cannot conceive of half a mind while we can always conceive of half of a body, however small.

e. Descartes argues that if a foot were amputated, i.e. , the body has been shown to be divisible but nothing has been taken away from the mind.

f. Is it the case that the mind is indivisible? It can be argued that in the case of brain damage, there may be a loss of mental faculties; for example, a patient might lose part of memory, or the capacity to smell; think of the effects of dementia on mental capacity. If we consider the mind to be a set of faculties or capacities, then it does seem that we can lose some and that, therefore, the mind is in this sense divisible. Remember that Descartes thought that the mind was a completely separate entity from the brain; modern science has strongly implied that, whether dualism is true or not, what happens in the mind is very closely related to what happens in the brain. If the brain is damaged in certain ways, the mind is diminished in certain ways.

g. We frequently experience inner conflict. For example, we are tempted (by the cream cake, by the wish to drive fast, by the wish to be lazy) and we desire to resist temptation (to become slim, to be safe drivers, to work hard). This conflict implies that there are separate entities within us, that the mind is not one homogeneous whole but a set of different parts.


a. Reference: Meditation VI : SUMMARY: I know that whatever I clearly and distinctly perceive as separate (or as two different things) can be created by God as separate (or as two different things). I know that I exist and I clearly and distinctly see that nothing else belongs to my essential nature except that I am a non-extended thinking thing. On the other hand I have a distinct idea of my body as an extended, non-thinking thing. From this I am certain that I am really distinct from my body and can exist without it.

b. Objections are similar to those to the argument from doubt. Cottingham uses the example of a triangle. I can clearly and distinctly perceive that triangle ABC has the property of being right-angled without clearly and distinctly perceiving that it has the Pythagorean property, namely, that the square on its hypotenuse is equal to the sum of the squares on its other two sides. On Descartes’ reasoning the two properties would be distinct and one could exist without the other. But despite my perception that the one property is quite different from the other, they are in fact essentially linked. Even God could not create a right-angled triangle which lacked the Pythagorean property.

c. Descartes may have successfully shown that he is a thinking thing but this argument does not show what does the thinking might not be corporeal, i.e. , part of his body.

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