Could you exist without your body? Could you continue to think without your brain? René Descartes is famous, or notorious, for arguing that the answer to these questions is ‘yes’. In his first published work, the Discourse on the Method, which appeared anonymously in 1637, he observed that ‘this “I” [ce moi] … is entirely distinct from the body, and would not fail to be whatever it is, even if the body did not exist’.

In one way this was a surprising position for him to take. For Descartes was in many respects what we should now call a scientist. He was fascinated by physiology, and when he lived in Amsterdam in the late 1620s he regularly obtained carcasses for dissection from his local butcher, so as to advance his research into the physical mechanisms responsible for animal and human behaviour. His project was to explain a whole range of familiar phenomena (including respiration, digestion, growth, physical movements and behavioural responses) by reference to the mechanical operations of the internal organs. Descartes compared the body to a ‘clock or an artificial fountain or mill’ – a machine operating purely in accordance with the mathematically describable laws of particle physics. So far from needing to invoke any immaterial ‘soul’, Descartes insisted that the same physical laws operate in the biological realm as apply to any other part of the universe.

So one might perhaps have expected Descartes to go all the way and become a thoroughgoing materialist, reducing everything to physical mechanisms. But his mechanical and mathematical schemas of explanation ground to a halt when it came to our human powers of thought and language. Descartes considered these functions to be just too complex, and too creative, to be performed by a machine. Machines, he argued, always need a fixed correlation between input and output, and hence no machine can function in the fluid, open-ended, universally adaptive way that our human reason is able to function. So, for Descartes, only a non-physical, divinely created, ‘rational soul’ can do the job. He sums up this argument in Part Five of the Discourse on the Method as follows:

Descartes thus believed that a non-material soul was needed to explain what physical science could not account for. It has to be remembered, though, that Descartes's grasp of the physical structures of the body was fairly crude by today's standards: he thought of the nerves, for example, as little pipes filled with vapour, and he had no conception whatever of the staggering neurological intricacy of the cerebral cortex. Though this is a speculative question, it's perhaps worth asking if Descartes might have hesitated over the need to introduce an immaterial soul to explain thought and reason had he been aware of the incredible complexity of the human brain, consisting, as we now know, of many thousands of millions of neural connections.

But in arguing for the existence of an immaterial ‘rational soul’, Descartes had other strings to his bow. In addition to his argument from the non-mechanical nature of thought, he also produced several independent arguments to show that the soul, or thinking self, must be entirely distinct from anything material. The first of these (in a passage in his Discourse on the Method from which the phrase in our opening paragraph is taken) is the argument from doubt:

Apply this to yourself. You can doubt the existence of your body – you can imagine yourself floating free of the body, or even perhaps continuing to exist and to think in some entirely non-physical realm; and hence your body can't be essential to what makes you you. So it follows that you can rightly affirm, just as Descartes did, that ‘this me by which I am what I am’ is ‘entirely distinct from the body and could exist without it’.

Is this argument from doubt a good one? On further reflection the reasoning seems dubious. Suppose that, being ignorant of chemistry, it is possible for me to doubt the existence of carbohydrates, and yet I cannot doubt that this potato in front of me exists. Does it follow that carbohydrates are not essential to what makes a potato a potato, or that the potato could exist entirely independently of carbohydrates? The example may seem whimsical, but it illustrates the perils of arguing from premises about what I know about something, or what I am capable of doubting about it, to conclusions about its true nature or essence.

The dubious argument from doubt is recapitulated in the course of Descartes's metaphysical masterpiece, the Meditations, published in 1641, a few years after the Discourse, and written this time not in French but in Latin (Latin being, in the seventeenth century, rather as English is today, the language of choice if you wanted to reach an international audience). But having repeated his argument from doubt in the Second Meditation, Descartes goes on, in the Sixth and final Meditation, to add two further arguments for the immaterial nature of the mind or soul. It should be noted in passing that Descartes, rather strangely to our modern ears, often uses the terms ‘mind’ and ‘soul’ interchangeably. (For those who are interested, the usage of these and other related terms in Descartes and other philosophical and literary authors is discussed in my recently published book In Search of the Soul.)

Descartes's arguments in the Sixth Meditation are designed to show that the thinking substance which we call the mind or soul belongs to an entirely different domain from that of physical reality. The first argument is based on our clear understanding of a thinking thing as something completely and utterly distinct from an extended thing:

Descartes calls the physical world (including the human body) res extensa, the Latin for ‘extended stuff’, or ‘extended substance’ (literally ‘extended thing’); and extended simply means having extension in three dimensions (height, breadth and depth). Res extensa, in short, is whatever has dimensions, and so can be measured. But thought doesn't seem to be mathematically quantifiable in this way. So, Descartes reasons, thinking substance (in Latin, res cogitans) must be totally different, different in kind, from any extended substance. Its essence or nature is entirely distinct from the essence or nature of anything quantitatively measurable.

Is this a good argument? One may reasonably have a suspicion that it begs the question. For if you ask what is the ‘thing’ or substance that is doing the thinking in any given case, the most obvious and reasonable answer would seem to be ‘a human being’. When thinking is going on, in your case or in mine, the thinking is surely being done by this or that human being, in virtue of activity going on in his or her brain. And if that is right, then then the ‘thinking thing’, the substance that is doing the thinking, is after all (despite what Descartes says) an extended thing – the three-dimensional biological creature we call a human being.

Descartes has a second argument for the non-bodily nature of the mind, which is closely connected with his argument about the difference between thinking and being extended, namely an argument from the divisibility of body:

To summarize: matter, extended stuff, is always divisible; mind, thinking stuff, is indivisible; so their natures must be utterly different. Is this a good argument? One might well question the second premise: are we prepared to go along with Descartes's claim that the mind is ‘entirely single and complete, without parts’? You do not have to accept everything that Freud later taught about the hidden parts of the mind in order to feel unconvinced when Descartes asserts that the mind is a simple indivisible unity. For even if we confine ourselves to fully conscious thought, we are surely often aware of conflicting elements, mutually opposed goals, clashes between impulsive desire and restraining reason and so on, which might make us want to question Descartes's confident insistence that ‘I am unable to distinguish any parts within myself’.

So as we come to the end of our survey of Descartes's various attempts to establish the immaterial nature of the mind, it seems hard to see any of them as fully persuasive.

* * *

I cannot think of any single philosopher, from Plato right down to the present day, whose arguments have not been subject to fierce criticism, so it is hardly surprising that Descartes's arguments for the immaterial nature of the mind have seemed to many to be open to question. What is perhaps more surprising is that Descartes himself shows frequent signs of discomfort about the stark opposition between mind and matter which he had set himself to establish. (This stark contrast between mind and matter has come to be known as ‘Cartesian dualism’ – ‘Cartesian’ from Cartesius, the Latin version of Descartes's name, and ‘dualism’ because he argues for two distinct categories of substance, mind and matter.) Descartes never retracted his dualistic thesis that mind and body are two entirely distinct substances; but it is striking that he went on to give many indications that the thesis was in serious need of qualification.

To see what led Descartes to qualify his mind–body dualism, or at the very least to add some careful annotations as to how it should be understood, it will be useful to approach things from the phenomenological angle, that is to say, by thinking about how things feel to the individual experiencing subject. As you go through life, following your daily routine, eating breakfast, sipping coffee in your study, taking a walk in the fresh air, and so on, do your experiences really feel like those of an immaterial mind or soul that happens somehow to be lodged in a physical body? The answer I should give, and I imagine most of those reading these words would give, is something like the following:

Now the crucial point to note here is that Descartes not only conceded the force of this kind of possible objection to his mind–body dualism, but he actually underlined its importance in arriving at a properly nuanced understanding of how mind and body are related. One of the key passages comes in the Sixth Meditation, as an immediate sequel to the very same discussion in which Descartes had argued for the distinctness of mind and body. It runs as follows (emphasis added):

Descartes later wrote to a correspondent that if an incorporeal mind or spirit (like an angel) were occupying a humanoid body, it would not have sensations like pain: ‘If an angel were in a human body, it would not have sensations as we do, but would simply perceive the motions [in its body] which are caused by external objects, and in this way would differ from a real human being’ (Letter to Regius of January 1642).

So the ordinary sensations and feelings I experience every day are signs that I am not a disembodied mind, but a creature of flesh and blood, a genuine human being, or, in Descartes's Latin, a verus homo (or, in the French he used elsewhere, un vrai homme). Recognizing the human being as a genuine entity in its own right means that Descartes's dualism receives a crucial qualification. For although ontologically speaking (in terms of what substances exist) Descartes recognizes only two types of substance, namely mind and body, nevertheless in terms of the types of attribute or property to be found in the world, it turns out that there are not two but three categories. (Here's an analogy: in the chemical compound we know as water, there exist only two elements, hydrogen and oxygen. But in addition to the properties of hydrogen on its own, and those of oxygen on its own, a third set of properties arises when they are compounded – ‘watery’ properties (like being good to drink) which are not properties of either element on its own.) So athough Descartes remains a substance dualist, he might also be called a ‘property trialist’ – he believed there were three distinctive types of property or attribute.

The three types of property or attribute, according to Descartes, are as follows. First (as we have seen) there is the attribute of extension which belongs to, and defines, matter or physical stuff: matter is whatever is extended in three dimensions, length, breadth and height, and so can be mathematically quantified. Second, there is the attribute of thought, which (again as we have seen) belongs according to Descartes to an immaterial mind, or ‘thinking thing’; and the two chief modes of thought are, Descartes went on to explain, understanding and willing. But there is for Descartes a third set of distinctive attributes, comprising feelings, emotions, sensations and passions, which belong neither to the body on its own, nor to the mind on its own, but which arise from what Descartes calls the ‘intermingling’ of mind and body. Feelings (like hunger, or thirst), sensations (like giddiness, or itching) and passions (like jealousy, anger, shame, or love) – these are the signatures of our genuine humanity, signs that we are not mere ghostly minds in mechanical bodies, but are embodied creatures of flesh and blood.

The passions we experience as ‘genuine human beings’ were seen by Descartes as crucial for the ethical quality of our lives. In his last published work, The Passions of the Soul, written shortly before his ill-fated visit to Sweden in the winter of 1649–50, where he died of pneumonia aged fifty-three, Descartes remarked that life's greatest pleasures are reserved for ‘those whom the passions can move most deeply’. And what matters from the ethical point of view is not just the physiological basis of the passions (vital though this is for understanding how we work), but the way in which such events are experienced by the conscious subject as fear, hope, anxiety, confidence, despair, jealousy, pity, anger, pride, shame, cheerfulness and love. Here Descartes offers the hope that by careful training we can achieve genuine human fulfilment – a fulfilment that does not deny our biological inheritance. The pleasures which belong to us as human beings, the pleasures which do not belong to ‘the soul apart’ but are ‘common to soul and body’, depend, wrote Descartes, ‘entirely on the passions’, and enable us to taste the ‘greatest sweetness that life has to offer’.

Where do Descartes's complex views on the relation between mind and body stand today? What has crucially changed since Descartes's time is the massive scientific progress enabling us to identify the role of the cerebral cortex in human mental processes; and as result of this, few philosophers would now go along with Descartes in ascribing the ‘pure’ modes of thought, understanding and willing, to a wholly immaterial mental substance. On the contrary, in the light of what we now know about the brain, it seems clear that the body is intimately involved in all conscious processes, even the most abstract intellectual thought.

Given the pervasive involvement of the body and the brain in consciousness, the majority of philosophers today have little time for Cartesian dualism, or for Descartes's immaterial mind or soul. But even if we go along with this rejection of these major elements in Descartes's view of the mind, something of his legacy, I would suggest, still survives, and still has something to teach us. Indeed, it is striking how the problems Descartes bequeathed to philosophy still live on in one form or another. Although we may reject Descartes's idea of mind and body as quite distinct and incompatible substances, the precise relation between mental and physical phenomena continues to perplex today's philosophers. Many, for example, are preoccupied with the so-called ‘hard problem’ of consciousness, the problem of seeing how purely physical events, such as the firing of neurons in the brain, can give rise to the subjective awareness that each of us experiences in our conscious lives. What is more, even if all human thought has turned out to depend intimately on physical and biological processes in the body and the brain, it nevertheless seems clear that it is not entirely reducible to those processes. For even if someone were to have a complete ‘print out’ of all the physical processes going on in the body and the brain at any given time, this would still not yield an understanding of what it is like for someone to have the corresponding experiences; nor would it yield an understanding of the meaning of the relevant thoughts and feelings.

So even if we reject Descartes's view of the mind as a distinct, non-physical substance, it remains true that conscious thought cannot be fully explained in physical terms. Descartes's ghost continues to haunt us. And whatever final verdict you may reach on his philosophy of mind, there can be no doubt that to study Descartes's absorbing and brilliantly clear writings on the enduring enigma of the human mind is to realize that he was one of the greatest and most original thinkers of all time, and that he continues in many ways fully to deserve the title that has so often been applied to him – ‘the father of modern philosophy’.