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Descartes Arguments for Substance Dualism

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Does Descartes provide a convincing argument for the claim that mind and matter are distinct substances Descartes’ Argument For Dualism

In his Meditations Rene Descartes aimed to reconstruct the whole of science by trying to prove the distinction between mind and matter. He gives an argument from doubt, and another from conceivability. I will give a brief summary of the foundations Descartes builds his thesis on, and then looking at his arguments and whether they are capable of persuading us that dualism is a logical stance to hold.

To what extent if any is Descartes successful in showing there is a real distinction between mind and body

In Descartes’ Meditations, Descartes aims to reshape the whole of science by starting from foundations that can be deductively proven. I will briefly summarise and criticise the important parts of the meditations on which his dualist argument rests and then go through each of the arguments that he raises in order to prove the distinctness of mind and body and critique each of them. I will focus on the logic behind his arguments, finding holes in his strategy and places where he fails to prove the next step. With this I will show that Descartes is not successful in showing that there is a real distinction between mind and body.

In the Meditations, Descartes aims to find a firm foundation for knowledge, to find indubitable knowledge, to refute scepticism and vindicate rationalism, and to prove the existence of God. Though a rationalist himself, Descartes assumes a sceptical approach when considering what we can be certain of. He quickly rejects a priori and a posteriori knowledge, concluding in Meditation 2, that all he can be certain of is his own existence in some form. From here he uses an ontological argument to affirm the existence of a perfect God. Using these foundations builds an argument to reconstruct science, and to show that “it is certain that this I is entirely and absolutely distinct from my body, and can exist without it.”

In Meditations, he begins by first rendering any thought or concept, of which there can be doubt, negligible so that all he would be left with is things that are unquestionably true. The first thing that he proves is the fact that he exists. He shows that as he is thinking, he must necessarily exist in some form. From this he proves the existence of God using an ontological argument and from both of these foundations he aims to derive science as a whole.

Argument from Essence
In Descartes’ second meditation, he shows that although he knows that he exists, he wants to first work out what it is that he is (AT 7:25-28) . He comes to the conclusion that his essence is thinking. He is a thinking thing. But does his body constitute a part of his essence too? Descartes uses the following argument to show that his body is not a part of his essence.

– If I was a body then doubts about my body would be doubts about my existence – I doubt my body as it cannot be conceived clearly and distinctly – I do exist as proven by the Cognito

– Therefore I cannot be a body.

One can clearly and distinctly think of the mind as something that thinks and is unextended (immaterial), whereas the body is extended and cannot think. This means that one can conceive of a thinking thing without extension i.e. a mind without matter. According to Descartes earlier meditations, this means that God could have created made them as distinct. Therefore, they must be distinct as if they weren’t, not even God could create them that way (AT 7:121). The problem here is in the fact that Descartes has presumed his essence is to think. He has proven that he does exist and he does think. His argument for his essence to be thought itself however is limited by his subjective point of view. One can see that if we imagine an inanimate object such as a table begins to think, the table itself could come to the same conclusion as Descartes through the cogito that it is a thinking thing. Then one could see that God could take away that ability to think, and it would still remain a table. It’s essence is not to think. We can then use this argument to see that it may be that the body is part of one’s essence but we cannot know this a priori. We have to be able to take an objective point of view (AT 7:198). If one takes the above response to Descartes’ argument, one can see that Descartes is guilty of some small circular reasoning. He assumes that the mind is immaterial itself and its only essence is thinking. This cannot be proven. Yet, we can see how the mind in modern times may be explained from a materialist point of view. Therefore as the first premise cannot be proven, the argument is no longer valid.

Argument from Conceivability

In his sixth meditation Descartes provides the Argument from Conceivability to convince us of dualism: It is conceivable that I exist in thought, without my extended body also existing. Moreover it is possible that I, a thinking being, can exist without my extended body existing. If it is logically possible that I can exist without my body existing then they are not identical and thus separate entities. Therefore I, a thinking being, am not the same as my extended body.

The Argument from Conceivability falls short on soundness – conceivability is not sufficient for logical possibility. For example, in number theory one of the greatest unsolved hypothesis is Goldbach’s conjecture. That is that every positive integer greater or equal to 4 can be expressed as the sum of two primes. So though I may be able to conceive of some number being discovered that is not the sum of two primes, if the conjecture is necessarily true (proven or otherwise,) I will have conceived of a logical impossibility. Thus Descartes premises are false and fail to convince us of substance dualism.

Moreover, Descartes relies on having a thorough knowledge of mind and body. We may conclude with Descartes that thought is necessary to having a mind, and materiality is necessary to having a body, it does not inevitably follow that there is an entity whose sole nature is to think. Descartes is limited by his subjective viewpoint that it could not be the case that extension could be another property of mind. He needs to prove the stronger argument that it is not possible for the mind to have extension as one of it’s properties. Descartes tries to make this proof in his Divisibility Argument:

Similar to this is the Argument from Distinctness which argues for the logical possibility of separating mind from body. To do this, mind and body would have to be distinct and thus it would prove both things.

– I can imagine myself as a thinking thing existing apart from the body (as shown by the different essences in the argument from essence.). – It is conceivable that a thinking thing can exist without a body – It is possible that a thinking thing can exist without a body – A thinking thing is not a body

The first premise has some major issues if we refer back to our objections to the argument from essence. We came to the conclusion that Descartes has not satisfactorily proven the fact that a thinking thing can exist outside the body as he jumps from the fact that he thinks to the fact that his whole essence is thinking without any extension. It seems very hard to think of an immaterial substance with causal force existing on its own without matter. If this is so the argument fails on premise one.

If the first premise is taken to be true, then there are still issues with this argument. It rests on the fact that if two objects are the same object then you cannot have one without the other. This is a logical truth. However, it seems that the fact something can be imagined is not a reliable premise for possibility. There are colours past our range of sight that we cannot imagine which are possible and similarly there are things such as Lois Lane could imagine Clark Kent without Superman. However, this is impossible as they are the same person. These examples are epistemic possibilities and not metaphysical ones, so one has to work out what type of possibility is shown in the argument using mind and body. To work this out, one would have to see objectively a posteriori if it is possible which in itself is impossible to do whilst living. If one dies and it is separated, the mind is immaterial and thus it seems intangible. So we cannot prove it right or wrong. Argument from Doubt

Descartes’ argument from doubt can be summarised as follows: It is possible for me to doubt the existence of everything material. However, I cannot doubt the existence of my own mind, as denial would be paradoxical. Therefore, my body and mind are not identical but are distinct from one another. This argument from the discourse on the Method

This argument for dualism is unconvincing as it is logically invalid. For example, though David Cameron may be able to claim uncertainty as to whether the Prime Minister exists, he can be certain of his own existence. From this it does not follow that David Cameron and the Prime Minister are distinct beings.

To resolve the invalidity of Descartes’ argument, a new premise can be added –Leibniz’s Law of the Indiscernibility of the Identicals. That is, if any two objects are identical, they are one and the same, they share the same properties. However even by adding this new premise, the argument is still unconvincing as it is assigning a different type of property to doubt, than the kind of property Leibniz is referring to. For example, in Greek mythology Oedipus wanted to marry Jocasta although he had no desire to marry his mother. It would seem that the Argument from Doubt is an epistemic claim rather than a metaphysical one, enlightening us more to our way of thinking rather than the nature of reality.

In the discourse on the Method, Descartes argues using Leibniz’s Law of the Identity of Indiscernible which states that two objects are identical if and only if they have all the same properties. – As I doubt that my body exists

– And I do not doubt that I and my thoughts exist – Therefore I am not identical to my body

This argument works because the property of being doubted does not apply to both thoughts or the mind and the body. Leibniz’s Law states that two objects are identical if and only if they have all the same properties. According to the preceding statement of the law, this means that mind and body cannot be the same thing. This is an argument that is easily refutable as one can see that being doubtable is not a property of the object itself. If I doubt something, the object does not change. It is actually a statement about the person who is meditating on this subject, an epistemic claim and not a metaphysical one. Due to this, one cannot apply Leibniz’s law to the first two premises and the argument is invalid. The Mark of Truth

Much of Descartes’ arguments for the distinct nature of mind and body rely on his previous argument in third meditation for the “mark of truth” (Hatfield). This states that whatever he clearly and distinctly perceives is true as his perfect God would not allow for such deception. As Gassendi argues (AT 7:122), we are not clear as to what the guiding tenets of clear and distinct knowledge are as we often are adamant about the truth of a fact, we believe our thought is both clear and distinct, however we are mistaken. For example, Newtonian Physics which seems to be understood clearly and distinctly by Newton which was later disproved by Einstein. Descartes replies in saying that he accepts the need for a good way of measuring clear and distinct thought but that the outcomes will always be unable to be doubted. The affirmation of the fact and the thought cannot be made otherwise. It would seem he is saying that it would have to be a logical truth. How can one clearly and distinctly know anything about the real world other than one’s own existence, mathematics and logic?

It seems that much of Descartes logic is flawed in that it either commentates on human knowledge and personal doubt and not about the object or it seems to presuppose the distinctness of mind and body to begin with. If we exchange the concepts, it seems to bring about things that seem instinctively false. Furthermore, the argument from conceivability and essence both rely on the mark of truth and thus the existence of God, both of which have questionable logic themselves. Therefore, Descartes has not been successful in any way in showing the distinct natures of mind and body.


Descartes, RenĂ©. Oeuvres de Descartes, 11 vols. Ed. Charles Adam and Paul Tannery. Paris: Vrin/CNRS, 1964–76.

Hatfield, Gary. René Descartes. Ed. Edward N. Zalta. Summer 2011. 25 February 2012 .

Robinson, Howard. Dualism. Ed. Edward N. Zalta. Winter 2011. 26 February 2012 .

Wilson, Catherine. Descartes’s Meditations an introduction. Cambridge: University Press, 2003.

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