28. Two of Descartes’ Arguments for Dualism

In his Meditations (1641), Rene Descartes argues for a version of metaphysical dualism that maintains the mind and body are two completely different types of entities.  The mind is not physical and so is not extended, doesn’t take up space, and is not divisible as physical matter is.  The body is physical and so is extended, takes up space, and is divisible.  The mind also seems to have properties that the body doesn’t have: the mind can will, think, imagine, set goals, entertain reasons and sense the world around it.   Perhaps more importantly, the mind is the seat of free will whereas the body is determined by laws of nature.

We can clarify Descartes’ strategy with the help of a principle that was introduced by the rationalist philosopher Gottfried Wilhelm Leibniz (1646-1716). One of Leibniz’s most famous principles, The Identity of Indiscernibles (sometimes referred to as Leibniz’s Law of Identity), is particularly useful here.  The principle is as follows:

  • The Identity of Indiscernibles: Two things are identical if, and only if, they simultaneously share exactly the same properties. One can also formulate it more logically: If, for every property F, if object x has F if and only if object y has F, then x is identical to y.

Many philosophers who support dualism use this principle and try to identify attributes of mind that are lacked by matter.  For, according to the principle, if mind and matter are the same then they should have the same properties.  Thus discovering a property of mind that is not shared by matter would establish dualism in some form. After all, if they are not identical then they are not one: they are two different entities.

According to Descartes, the mind has a property that the body does not: the mind is indivisible. He writes: “Now my first observation here is that there is a great difference between a mind and a body in that a body, by its very nature, is always divisible.  On the other hand, the mind is utterly indivisible” (56)[1].  He goes on to argue that willing, sensing, and understanding are not parts of the mind since “it is one and the same mind that wills, senses and understands” (56). This seems to make sense: do we really have several pieces or parts of anger, a belief, an imaginative vision, and a will towards something?  We can speak of parts of the brain. But can we divide up the mind and its activities?  Descartes says no. If this is the case then, according to Leibniz’s Identity of Indiscernibles, the mind is not reducible to the physical brain: they are two different entities. His first argument can be formulated as follows:

Premise 1: The mind is indivisible by its very nature.

Premise 2: The body is divisible by its very nature.

Premise 3: Two things are identical if, and only if, they simultaneously share exactly the same properties (Leibniz’s Law).

Conclusion 1: My mind has a property my body doesn’t have, namely, being indivisible.

Conclusion 2: Therefore, the mind and body are not identical—they are two different things (dualism is true).

One might certainly object that premise 1 needs to be justified.  One way to do so is to focus on the fact that we often experience multitudes of sensations, ideas, and words as unified and as ours. In most cases we don’t just hear individual words in succession but meaningful sentences and a flow of a conversation; we don’t just experience random colors, sounds, and shapes but coherent objects; we don’t just think disjointed ideas but grasp networks of ideas as theories, arguments, and implications. But how do these forms of unification take place?  Well, we could try and account for them physically and discuss how matter can function in ways conducive to unity.  However, some philosophers have pointed out matter cannot give rise to true unities since it is complex, made of parts, and infinitely divisible. Consider this passage from Leibniz:

“Since every extended body, as it is really found in the world, is in fact like an army of creatures, or a herd, or a place of confluence, like a cheese filled with worms, a connection between the parts of the body is no more necessary than is a connection between the parts of an army. And just as some soldiers can be replaced by others in an army, so some parts can be replaced by others in every extended body. Thus no part has any necessary connection with any other part, even though it is true of matter in general that when any part is removed, it must necessarily be replaced by some other part, just as it is necessary, when soldiers are confined in a small enclosure or one which will not hold many, for another to take the place of anyone who goes out.”[2]

If matter has no natural interconnection then perhaps it can’t truly unify sensations, ideas, and words.  Perhaps only a true unity is capable of truly unifying multitudes. And this true unity would be the indivisible and therefore truly unified mind. perhaps a simple mind not made of parts would be necessary to perceive or make unities out of the multitudes of sensations and thoughts we experience. One modern version of this argument, known to many as the “narrow Achilles of rational psychology”, is as follows:

Premise 1: Unification of representations takes place

Premise 2: Only a simple, unified substance can unify representations.

Conclusion: Therefore, the human soul or mind is a simple unified substance.[3]

The belief that we are essentially a simple, non-physical substance has been very popular in the history of philosophy and rightly so: most of us think we are, in some fundamental sense, the same person over time.  Descartes’ notion of a simple substance would be that fundamental something that remains underneath all the changes.  This vision is important when it comes to our practical lives: we judge others for things they have done in the past as if they are the still the same person; we put people in prison based on what they have done in the past as if they are the same person.  It seems like responsibility, regrets, remorse and many other human feelings are made possible by their being a self that remains the same over time.  Since matter in motion is not the same from moment to moment, some think a dualistic vision of ourselves can ground very common intuitions about our self-identity over time and the moral life that is made possible by it. Moreover, this simplicity of the soul can help us argue for immortality as follows:

Premise 1: If something can die that something must be made of parts that can eventually disintegrate.

Premise 2: My mind, unlike my body, is simple, i.e., not made of parts.

Conclusion: So my mind cannot disintegrate and die.

In any case, there is another property Descartes presents that can help establish dualism. In the First Meditation, he claims he can doubt the existence of the body using his method of doubt.  However, in the Second Meditation, he argues that he cannot doubt his own existence as a thinking being.  After all, if he is doubting then such an activity assumes he exists as someone to doubt: to raise objections, entertain thought-experiments, etc.  So he cannot doubt his own existence because to do so would end up proving it.  This leads him to claim that the proposition, ‘I am, I exist’ is certain every time he utters it.  Now given these claims, we can apply Leibniz’s principle yet again and argue as follows: the body has a property that a thinking mind does not have, namely, the property of being doubtable. If this is the case then mind and body are two separate entities.  The formal argument is similar to the one above:

Premise 1: The mind is not doubtable by its very nature.

Premise 2: The body is doubtable by its very nature.

Premise 3: Two things are identical if, and only if, they simultaneously share exactly the same properties (Leibniz’s Law).

Conclusion 1: My mind has a property my body doesn’t have, namely, being undoubtable.

Conclusion 2: Therefore, the mind and body are not identical—they are two different things.

So those are Descartes’ two arguments for dualism.  What can be said against these claims?



[1] Rene Descartes, Meditations on First Philosophy (Indianapolis: Hackett, 1993).

[2] Nicholas Rescher, G. W. Leibniz’s Monadology: An Edition for Students (Pittsburgh: University of Pittsburgh Press, 1991), p. 50.

[3] See The Achilles of Rational Psychology, eds. Lennon and Stainton (Springer Publishing, 2008), p. 3.  The book includes a whole chapter that examines the Achilles argument in Leibniz (chapter 10).  The argument was named “Achilles” by Immanuel Kant.  The name suggests that this argument, like Homer’s Achilles from the Trojan War, is indeed a powerful one and perhaps the champion of those who would rationally argue for the existence of the soul. But, like Homer’s Achilles, it has a fatal flaw that will bring it down: from the fact that there needs to be a unified subject of experience to represent a multitude, it doesn’t follow that this unified subject is a substance that is simple, will survive death, etc.  Kant sets out the fallacy as follows: “That which cannot be thought otherwise than as a subject does not exist otherwise than as subject, and is therefore substance. A thinking being, considered merely as such, cannot be thought otherwise than as subject. Therefore it exists also only as subject, that is, as substance.” See Immanuel Kant, Critique of Pure Reason (New York: St. Martin’s Press, 1965), p. 371.

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