The real distinction between mind and body is one of two claims celebrated in the book’s subtitle – in which are demonstrated the existence of God and the distinction between the human soul and the body (p12) – which indicates its central importance to Descartes’ project. He accordingly offers a number of arguments to establish that distinction. The first appears in the Second Meditation in his reflections on the Cogito, but is incomplete. After demonstrating the existence of the material world in the Sixth Meditation, that argument is developed further, and a second argument is offered.
At an early stage in his investigation, shortly after establishing the fact of his own existence through the cogito, Descartes describes his self, his nature, as essentially mental:
I am, then, in the strict sense only a thing that thinks; that is, I am a mind, or intelligence, or intellect, or reason—words whose meaning I have been ignorant of until now. But for all that I am a thing which is real and which truly exists. M2 p18
The Argument from Doubt, as it is known, is more implicit than explicit and emerges from this discussion of the true nature or essence of things. The essence of something consists of those properties the thing needs to be what it is. To borrow a common example, consider a big red triangle. Would it still be a triangle if it was blue? or if it was smaller? Clearly yes, in both instances. So it seems that colour and size are not essential to a triangle: both can change and the triangle will remain. Would it still be a triangle if it had four sides? Of course not: it would be a quadrilateral. So having three sides is an essential part of being a triangle. Putting it another way, colour and size can be logically separated from a triangle; having three sides cannot (try to imagine a four-sided triangle).
This is how Descartes proceeds to investigate his own nature. He knows that he is at least a thinking thing: the Cogito has shown that thinking is essential to his existence (‘... thought; this alone is inseparable from me’ p18). Essential properties cannot be subtracted from a concept and the concept remain, as with the three sides of the triangle. But Descartes has already imagined that his body does not exist through the hypothesis of the malicious demon at the end of Meditation One:
I shall consider myself as not having hands or eyes, or flesh, or blood or senses... etc M1 p11
So if his body is logically separable from Descartes, then it is not essential to what Descartes is. And if it is logically possible that his mind exists and his body does not, then these must be two distinct things. As Descartes has already surmised, it appears that logically the mind could exist without a body:
I am not that structure of limbs which is called a human body. I am not even some thin vapour which permeates the limbs—a wind, fire, air, breath, or whatever I depict in my imagination; for these are things which I have supposed to be nothing. M2 p18
The argument can be formalised as follows:
This constitutes Descartes’ first attempt to distinguish the mind from the body. The argument is certainly open to criticism, although we should remember that at this stage of his investigation, Descartes is proceeding slowly and his aims are modest. A fuller defence of mind-body dualism will be presented in the Sixth Meditation (when Descartes has a clearer conception of res extensa, and a divine guarantee that his conceptions are true).
We should nevertheless note a characteristic problem with the Argument from Doubt. We can identify two different definitions of essence in play here: the objective sense that an essence is what a thing needs to be what it is (like the triangle’s three sides); and the subjective sense that an essence is what you cannot conceive a thing as not having. This subjective, psychological sense is what Descartes is employing: but, of course, there is no reason to think that the world must objectively conform to Descartes’ conceptions. Sometimes these two definitions coincide, as with the triangle: a four-sided triangle is inconceivable, and a triangle by definition must have three sides. But in Descartes’ case, being able to conceive of his mind as separate from his body does not entail that they are, in fact, distinct substances. This problem is characteristic because Descartes has restricted his enquiry to what he can conceive: certainty for Descartes is a psychological fact. For the claim of essential natures to be more convincing, we would expect an objective, scientific account of the nature of mind and body – but that is not available to Descartes at this point in his investigation.
In the Sixth Meditation Descartes establishes the existence of material things, and is thus in a position to reflect upon the difference between matter and mind. His argument extends the approach of the Second Meditation, but now he has the divine guarantee that his clear and distinct perceptions can be trusted. He deploys this to establish more securely the ‘real distinction’ between mind and body.
The Conceivability Argument recapitulates several aspects of the Argument from Doubt. By reflecting upon the true nature of mind and matter, Descartes concludes that – even though they are combined in him – they are logically separable.
... the fact that I can clearly and distinctly understand one thing apart from another is enough to make me certain that the two things are distinct, since they are capable of being separated, at least by God. M6 p54
His own nature has already been established as res cogitans – a conscious and non-extended substance. These properties are not logically separable from the nature of mind and are therefore essential. Material objects, however, are not conscious and are by definition extended – the nature of matter is to occupy space. With different natures, they therefore must be different substances:
... on the one hand I have a clear and distinct idea of myself, in so far as I am simply a thinking, non-extended thing; and on the other hand I have a distinct idea of body, in so far as this is simply an extended, non-thinking thing. And accordingly, it is certain that I am really distinct from my body, and can exist without it. M6 p54
The argument can be formalised as follows:
The argument makes implicit use of a principle that would become known as Leibniz’s Law. This states that if two things are in fact the same, then what is true of one must also be true of the other; or, conversely, if two things possess different properties, then those two things cannot be the same. In this case, since the mind thinks but does not occupy space, it cannot be the same substance as matter which does not think and does occupy space. If mind and body were the same substance, they would share all the same properties.
At first glance, it appears that Descartes has misapplied Leibniz’s Law. His perception of mind and matter does not count as a property of mind and matter – just because he perceives mind as unextended (for instance) does not entail that it is truly unextended – he could be mistaken in this respect. Applying Leibniz’s Law in an intensional context like this leads to what is known as the Masked Man Fallacy. Consider the following argument:
Of course, the masked man could well be my father – I assume the two men are different based on my knowledge of them, rather than any objective facts. It appears that Descartes has committed something like this fallacy in the Conceivability Argument. Descartes, of course, would deny this: his perception that mind is unextended is not taken as a property of mind, but as proof that mind is – in fact – unextended.
Notwithstanding his trust in clear and distinct ideas, the fact remains that conceivability is not quite the same as logical possibility: just because Descartes conceives mind and body as separable, does not entail that they are different substances. Perhaps, as Spinoza would claim, they are two aspects of a single substance. And even if Descartes can be certain that mind thinks, he cannot be sure that matter does not (in fact, it probably does). He appears to slide illegitimately from ‘I do not know that the mind has a physical element’ to ‘I know that the mind does not have a physical element’.
This is the objection raised by Antoine Arnauld. He gives as an example a man who recognises a right-angled triangle but does not know Pythagoras’ Law:
‘I clearly and distinctly perceive’, he may say, ‘that the triangle is right-angled; but I doubt that the square on the hypotenuse is equal to the squares on the other two sides; therefore it does not belong to the essence of the triangle that the square on its hypotenuse is equal to the squares on the other sides.’ OR4 p109
Similarly, Descartes may correctly perceive that his nature is to think, but he may be mistaken in believing that it is solely his nature to think. The fact that he conceives mind and body as separate substances is therefore no guarantee that they in fact are separate substances. Descartes’ reply to this is interesting. As Arnauld anticipates, he claims that the mathematician in the example does not have a clear perception of triangles if he fails to recognise the necessary relation implied in Pythagoras’ Law. More importantly, he rejects the analogy on the grounds that he is speaking of distinct substances – and their logical separability is essential to the concept of substance:
... the notion of a substance is just this—that it can exist by itself, that is without the aid of any other substance. And there is no one who has ever perceived two substances by means of two different concepts withut judging that they are really distinct. OR4 p11
The third and final argument for the real distinction between body and mind appears shortly after the Conceivability Argument. Here Descartes appeals to divisibility as a property that is possessed by bodies but not minds:
... there is a great difference between the mind and the body, inasmuch as the body is by its very nature always divisible, while the mind is utterly indivisible. M6 p59
By Leibniz’s Law, therefore, the mind and the body must be distinct. It is important to note that in the passage just quoted, Descartes refers to the ‘very nature’ of mind and body. Shortly after this, he describes his conception of the mind – ‘I understand myself to be something quite single and complete’ (emphasis added). Since this phrasing involves Descartes in the Masked Man Fallacy, the more charitable interpretation of the Divisibility Argument is to understand it as a metaphysical claim about the nature of things. In this form, it appears to be valid.
But philosophers generally do not accept that it is sound, because one of the premises is false: namely, that the mind is strictly indivisible. At first glance the claim sounds plausible: certainly, I experience myself as a unitary individual and it is difficult to imagine my self as divided. But this is only my experience: certain forms of schizophrenia appear to deliver a very different experience. Given the dependence of the mind on the brain, it may be the case that a divided brain will generate a divided mind – and that is exactly the conclusion drawn from split-brain patients. Moreover, Thomas Nagel’s analysis of these cases leads him to doubt ‘that there is anything very important that we are one of’ (Nagel p164), an expression which recalls David Hume’s claim that we are only bundles of perceptions. A bundle, of course, is eminently divisible. More generally, the notion that the mind is a separate substance no longer has a place in modern thought, rendering Descartes’ argument void.
Having established the metaphysical distinction between mind and body, Descartes must also account for the relation between them. This is far from an easy task: Descartes has outlined two fundamentally different substances with exclusive properties. The body is a machine: extended, divisible, inanimate, indistinct and subject to deterministic physical laws. The mind is unextended and indivisible, essentially conscious, transparent and free.
Nevertheless, mind and body interact: everyday experience demonstrates the effect of our thoughts on our actions, and vice versa. Descartes does not wish to deny this:
Nature also teaches me, by these sensations of pain, hunger, thirst and so on, that I am not merely present in my body as a sailor is present in a ship, but that I am very closely joined and, as it were, intermingled with it, so that I and the body form a unit. M6 p56
He notes that he has a special and intimate relationship with his body, a relationship that he cannot reproduce with other physical objects. When his body is wounded, he feels pain – although this does not have to be the case. He might have become only intellectually aware of the injury, like a machine running a diagnostic. Instead, the sensation of pain more effectively motivates him to avoid further injury. This particular combination of understanding and sensation is the best possible arrangement, although occasional malfunctions (like phantom limbs, or the thirst accompanying dropsy) do occur for purely mechanical reasons.
As ‘intermingled’ as the mind is with the body, the actual point of contact appears to be the pineal gland (see p59 fn), a delicate organ affected by the ‘animal spirits’ that convey information around the body. A wound in the foot, for instance, generates movement in the animal spirits which course to the brain and move the pineal gland, which in turn causes the mind to experience foot-pain.
This account would later be mocked by Gilbert Ryle as ‘the Ghost in the Machine’, and it does strike the modern mind as simplistic. Substance dualism has long fallen out of fashion. We should note, however, that there are still many philosophers who hold that no purely physical explanation can give an adequate account of consciousness. For as long as the ‘hard problem’ persists, we lack a full account of the purely mental aspects of our experiences.
Cartesian dualism, however, seems unavoidably to founder on the problem of interaction. Briefly, it seems impossible that an immaterial and unextended substance like mind could have a physical effect on a material and extended body. For two things to interact, they need to have something in common; a point of contact, a shared location in space, mass, energy – and so on. Without such a common property, it is impossible to understand how the mental can exert a force on the physical. Of course, modern physics gives us a comprehensive account of interaction between physical entities – but the account assumes a closed system, and would fail if it acknowledged non-physical contributions. A model which allowed for non-physical interactions would also appear to violate the law of the conservation of energy.
Modern science – more aware of the close correlation between brain states and mental states – recognises that in some sense the mind is the brain, and requires a corresponding physicalist account. Descartes’ mistake, however, is easy to understand: whatever the neuroscientific truth, we experience the world as divided creatures, and the intimate richness of our mental lives is one of the most peculiar facts in the world.