From the philosophical archive for the constructive study of ontological dualism:


by Sarah Broadie

quoted from: Proceedings of the Aristotelian Society (hardback), 102 (2001) 295

abstract Although they are often grouped together in comparison with non-dualist theories, Plato's soul—body dualism, and Descartes' mind—body dualism, are fundamentally different. The doctrines examined are those of the Phaedo and the Meditations. The main difference, from which others flow, lies in Plato's acceptance and Descartes' rejection of the assumption that the soul (= intellect) is identical with what animates the body.


When philosophy teachers present the '-ism's' pertinent to mind-body relations, and are still at the broad-brush stage, quite often one finds them pairing Plato and Descartes as the two most eminent dualists of our Western tradition. As Plato to the through-and-through materialist Democritus, so Descartes to Gassendi, it is often suggested—reasonably, perhaps. As the modern non-reductive materialist to his Cartesian bete noir, so Aristotle to Plato on soul-body relations, we are sometimes told—a misleading analogy, some think. For the purpose of contrast with various non-dualist views it may seem useful to group Plato's dualism and that of Descartes together, and in many contexts their differences may not matter. But if one simply compares the theories with each other, not with any third system, the differences are fascinating and seem important.

Of course there are similarities to sustain the initial pairing. Both philosophers argue that we consist of something incorporeal, whether one calls it 'mind' or 'soul', which for the time being is somehow united with a body that is part of the physical world. Both identify the self, the `I', with the incorporeal member of this alliance. Both hold that my mind or soul will survive the demise of the body by which I am now present to this audience— which in turn is present to me through its members' bodies. Both may be understood as holding that the mind or soul can exist altogether independently of body, though Plato may have changed position on this point.2 Both are concerned with the immortality of the soul.

Here I shall focus on separability of mind or soul from body in Plato's Phaedo and Descartes' Meditations. But first a word about terms. Several times already I have said 'mind or soul' as if the words meant the same, which of course they do not. Plato consistently speaks of the soul (psuche), but not so Descartes. In his preface addressed to the theologians at the Sorbonne Descartes claims that he will prove the immortality of the soul. He is using the church's label for the doctrine, but it is doubtful that what he thought he could prove is what the church means by the phrase. Roughly, I suppose, the church's meaning spotlights the human individual minus a biological body. It is this that can sin and be forgiven, is summoned to the Last Judgement, has prayers said for its salvation. But what Descartes believed he could show is the immortality of the mind or intellect, and although the mind, as he was for ever stressing, is prone to error and should be expected to conduct itself according to an intellectual code of conduct, its errors are not sins or offences against morality. In more philosophical contexts Descartes explicitly distinguishes mind from soul, reserving 'soul' for that which animates the body. In this sense of 'soul' he either denies that any such principle exists or reduces it to a physical configuration. The biological difference between a living body and a corpse is the purely physical difference between a machine in working order and one that is broken or worn out.

So what Descartes is left with, in addition to his machine-body—if his or any other body even exists, which at the beginning of the Meditations he calls into doubt—is a mind whose business is to think and imagine, but not to animate any corporeal system. And since it is himself that he finds thinking, and since he is unable, no matter how hard he tries, to doubt his own existence as this currently thinking thing, Descartes identifies himself with this mind. But at first he is not in a position to assert that he, or the mind that is he, can exist without the body, because prima facie it is possible that the mind's existence or its essential activity of thinking depends on body in some way. For even though the mind does not require body in the way in which an animating principle presumably requires a body if it is to do its thing of animating something, the mind may depend on the body in some other way, a way in which, so to speak, it is the body that gives life to the mind, much as an arrangement of particles gives rise to a magnetic field. Later on, however, Descartes maintains that according to his clear and distinct ideas of mind and body, neither of these natures contains or refers to the other. And meanwhile he takes himself to have established that everything he clearly and distinctly perceives is true. Hence he can conclude that mind, and perhaps soul in the theological sense, is separable from body, which is the basis for proving the mind or soul immortal.

Or, more precisely, Descartes can conclude that mind and body are separable from each other once he is free of his initial wholesale doubt concerning the real existence of body. For obviously if the physical world is only his finite mind's dream object, neither it nor any of its parts can exist independently of that dreaming. And in that case it may not be easy to show that the finite mind that dreams such a dream—a dream in which it is embodied and its body is part of a physical world—can be free of dreaming this or other dreams like it. But if we take the opposite hypothesis, that the physical world exists independently, then this world, especially the part of it that is Descartes' body, can reasonably be held responsible for the appearances of the physical that are present to Descartes' mind. In that case it is reasonable to assume these appearances will cease when body and mind actually separate. The mind will then be phenomenally unembodied as well as really so. But as long as it is uncertain whether the physical is real independently of the finite mind, one can suppose that either this mind generates the appearances from itself, or they are caused in it by God. But since the finite mind cannot be separated from God any more than it can be separated from itself, on either of these hypotheses the cause of the appearances is necessarily always with that finite mind—so why should it ever be without the appearances? It is true that in the sixth Meditation Descartes says he can clearly and distinctly understand himself to be a complete being even without his faculty of sensory and imaginational appearances. From this he concludes that he or his mind can exist without that faculty and its objects. It follows from this that those objects, the empirical appearances, arise neither from his own intellectual nature nor directly from God who is always present to, or even in, his mind. Thus Descartes is only one step away from concluding that the immediate source of these appearances must be something altogether different from mind, both from the finite mind that is Descartes himself, and from the infinite mind that is God. In sum, the source of the appearances must be a corporeal substance, a real physical thing that exists independently of Descartes' mind.

But let us stop our thinker before he takes that last step, and question him about his premiss. If he or his mind really is or would be a complete being minus the faculty of sensory and imaginational appearances, why, by his own admission, do these appearances beset him so? No doubt they fade away when he completely absorbs himself in pure mathematics or in thoughts about God and about pure finite mind, if there is such a thing as pure finite mind. But in Descartes' own experience the empirical appearances always return. So perhaps it is the nature of his mind to conjure them up for itself again and again, or to become receptive again and again to these effects caused in him by God. If, on careful reflection, one can consider this possible, Descartes is mistaken in claiming that the human mind can attain a clear, distinct and complete idea of itself as existing free of empirical appearances to itself. That these sometimes recede when the mind is abstractly engaged does not prove that they are not among the objects natural to it or naturally served up to it immediately by God. For where is it written that all the mind's natural objects are present to it at once? Certainly, Cartesian doubt can save Descartes from regarding these appearances as anything more than phenomenal, but he knows from experience that doubt cannot put an end to the phenomena as such. He may always be saddled with them, then, even if only as appearances recognised as such. In this sense, a sort of phenomenalist sense, the self's body and physical environment may be as immortal as the human mind.

Thus Descartes' ideas of himself or his mind are not, I think, able to show that the human mind is in every sense separable from body. To show this, he must fall back on the independent attractiveness of the thought that 'real' or 'externally' existing body is what causes the empirical appearances. This is of course an independently attractive thought to the extent that it is unattractive to suppose that God (whom Descartes has by now proved to exist and to be his creator) deceives or meanly frustrates a finite mind like that of Descartes. For insofar as Descartes cannot help taking the empirical appearances to be of independently existing bodies, if Descartes were always mistaken in this then God would be a deceiver; and even if Descartes can break out of the deception by means of systematic doubt, God would be cruel in making the escape depend on a method so hard for the human mind.

So if one is a Cartesian, the position that mind is separable from body, not only ontologically but also phenomenally, is secured by means of two conclusions: if there is any such thing as a really existing body, mind is not existentially dependent on it; and: body really exists and is the separable cause of mind's corporeal experiences.

I want now to say something about the universality of Cartesian separability, and something about what unites the separables while they are together. These are points on which Descartes and Plato differ fundamentally. First, universality: in claiming that mind and body are ontologically separable, Descartes, of course, claims much more than that a given mind can exist apart from a given natural body. Separability is guaranteed for him by the essence of mind in general and the essence of body in general. From his ideas of these essences he believes he can see that mind—any mind—can exist apart from body— any body, and vice versa (Meditation 6). This is in line with the Church's teaching, according to which every human soul comes to the Last Judgement either stripped of body altogether, or with a sort of supernatural body through which it can communicate and suffer, but which is not set in a natural physical environment and is not subject to the laws of physical nature.

Phenomenal separability, as I am calling it, is likewise universal for the Cartesian insofar as the Cartesian holds that mind as such is subject to corporeal appearances because and only because an associated real body causes them. It follows from this premiss that for any mind M, once the causal nexus between M and real body is broken, M is automatically separated not only from real body but also from all corporeal appearances.3

In sum: both ontologically and phenomenally, the possibility that a human mind is linked to corporeal things, and the possibility of its not being thus linked, flow from the nature common to all human minds; and a mind's actual linkage or non-linkage is or is based on its standing or not standing in causal nexus with something metaphysically external to itself. Its linkage or not to corporeal things is therefore not determined by any internal mental disposition of its own, still less by any internal respect in which one particular human mind may differ from another, for example in respect of strong involvement in a certain type of pursuit. Consider Descartes himself in his unusual if not unique enterprise of seeking certainty through doubt. This extraordinary practice can surely be described as a letting go of the corporeal perspective, and it leads him, or so he thinks, to the proof that mind and body are ontologically separable. But this proof applies even to minds sunk in ordinary habits of thinking, minds for which Cartesian doubt is meaningless and impossible. And this proof is not performatively given in the practice, but is derived from independent truths which the practice uncovers as suitable starting points. Thus what Descartes proves when he proves separation possible is a truth that would hold even if no mind ever engaged in Cartesian or similar detachment. It surely suits the doctors of orthodox theology that Descartes presents them with the discovery of a truth that is like the truths of logic and mathematics and Cartesian physics in that it holds no matter what any of us may think or feel about anything. This is by contrast with any facts or possibilities he himself might bring about through a mental activity willed by him.

Now for the question of what unites the Cartesian separables when they are together. It is not the finite mind's own agency that connects it with a body which it then feels to be its own. This could only be done by an act of will on the part of the finite mind. But although Descartes regards his will as 'not restricted in any way' (Meditation 4), its unrestricted domain turns out to consist entirely of propositions to which he may choose not to assent when they fail to be clear and distinct. This unrestricted will is not a will to bring anything about except its own assertion and denial of already constituted truths and falsehoods. For this unrestricted will belongs to Descartes insofar as he is pure intellect. On its own, therefore, it cannot take as its objects things that are sensed or imagined, for according to Descartes such things can be present to the mind only when it is already united with the body. Consequently, the explanation for this union cannot be that the finite mind wants or wills to be connected with a particular body, or with some particular body or other. For without sense experience we could not have an idea, either definite or indefinite, of a particular body. And presumably any explanation in terms of the mind's wanting to be connected with body would attend to what it feels like to have a body—the mind would be assumed to have a sense of what that feels like, and to be drawn towards a corresponding existence as if it would be at home in a body. But for Descartes such feelings and the imagination of them can only arise when the mind is already embodied, so they cannot explain embodiment.

Nor can we explain it by turning to body by itself. Obviously, body by itself is powerless to connect itself with a mind. Only God, a third being of infinite power, can cause by his will a union between substances of such mutually alien natures as mind and body. Of course every arrangement of finite things depends on the will of God, but other arrangements, say of body with body, fall within a natural system and can be explained by familiar secondary causes according to the system's laws. Mind and body, however, fall within no such single system, according to Descartes; their union therefore speaks directly of a supernatural cause. On present showing, this cause is as different from finite mind as it is from finite body, since the latter are both devoid of the third thing's power to unite them. In this respect, the finite mind is as passive and inert as matter is traditionally supposed to be.

Let me now turn to Plato.

Readers of the Phaedo sometimes take Plato to task for confusing soul as mind or that which thinks, with soul as that which animates the body. Perhaps this is a terrible mistake. But it is not a confusion in the sense of a blunder committed en route to something else. For the identification of thinking soul with animating soul is Plato's theory in the Phaedo.

In trying to understand this, one might seem to discern a close analogy between thinking and animating if one identifies thinking with the exercise of intelligence and assumes, as is natural for many people, that the practical sphere is the arena for exercising intelligence. For the person of practical intelligence is switched on to the practical demands of his situation in a way not unlike the way in which a perceptually sensitive organism is switched on to signals in the environment and its own body,4 and again not unlike the way in which the elements of a physiological system are switched on and off by chemical signals in the interest of purely biological animation. Again, someone who is irresponsive to things that interest most people may be said not to be properly alive, and even not to be properly animating his body. In saying this we need not mean that he functions below par physiologically; we may instead be regarding his body as a social presence, an instrument for action and communication, which comes to life when activated. Being alive on this level presupposes being biologically alive, and for most normal human beings, being biologically alive automatically results in life on the level of practice, except for when they are sleeping. These two modes of being alive are linked in such a way that, rather than deeming them analogous, one might, more primitively perhaps, fail to distinguish them, and thus conflate what thinks with what animates the body.

Plato's view, however, is quite different, because for him the paradigm exercise of intelligence is theoretical or at any rate not immediately practical: it deals in universals and abstractions, it is conducted at leisure from practical life, and it has no palpable effects except on the thoughts of oneself and a few interlocutors. Plato believes that the soul thinks best when dissociated from the body. He has two reasons: one is the observation that we cannot engage in the kind of thinking that for him is thinking par excellence when we are physically active and attending to goings on in our bodies and in our physical environment; and the other is his theory that the soul has latent within it a supremely pure and beautiful kind of knowledge which it could only have come by before birth into a body. Since the thinking soul is at its best when in full contact with the objects of this knowledge, Plato concludes that the best thing that can happen to this soul is to be separated from body upon death.

So far one might think that Plato's thinking soul cannot possibly be what animates the body; for it seems absurd to suggest that something both animates a body and is a pure intellect that functions best away from the body. But in fact, the belief that the soul is an intellect that functions best away from body is precisely one of two assumptions that lie at the base of Plato's equation of intellect with animator. The second assumption is that this self-same intellect is also intimately connected with the body. The argument for this is mediated by the concept of the self. On the one hand it is natural for Socrates and his interlocutors in the Phaedo to identify themselves with their intellects. After all, if you are Socrates and I am Simmias in the Phaedo, then what are you and I engaged in if not paradigmatic intellection, while minimally using our bodies to exchange our thoughts? If we could think at our best without ever exchanging thoughts, or could exchange thoughts by some non-physical means, then we as intellects would not need bodies at all. On the other hand, though, each one of us knows himself to be in or intimately connected with a body. And Socrates' friends know this of Socrates, or why would they dread losing Socrates once his physical death has been decreed? So the self that is Socrates' intellect is the self bound up with his body. And the fact that in this life the soul functions best as intellect when least involved in bodily activity and sensation, together with the doctrine that the soul's intellectual activity was at its absolute best when the soul was attached to no body, now strongly points to the conclusion that intellectual activity waxes as bodily involvement wanes and vice versa. And since it is natural to think of bare biological animation as the limiting case of a soul's bodily involvement, and as the basic form which more complicated forms—the ones expressed in actions and emotions—depend on and presuppose, it is not difficult to draw the further conclusion that the soul that can function as pure intellect is the same as the soul that keeps the body alive.5

But now if one and the same entity, the soul, can function both as unembodied intellect and as animator of a body, what determines it to one of these functions rather than the other? And since they are alternatives, and the soul is capable of both, is neither function essential to it, any more than a piece of wax is essentially the shape of a ball or essentially the shape of a cube? But if neither function is essential to the soul, we have been told nothing of the soul's nature. If, on the other hand, both are essential, what unites them?

According to the theory of the Phaedo, the soul becomes involved with a body because it desires to live in a way in which it only can if it has a body of suitable kind. To begin with, perhaps, the soul is not oriented to any very specific set of physical activities or pleasures, since it has no experience of any. So to begin with perhaps all that it takes to involve a soul with body is the soul's failure to understand or fully believe that its existence can be complete as a pure intellect. Not realizing this, it feels incomplete, and this breeds the desire for some non-intellectual activity; and lo and behold the soul finds itself with a body, and presumably a physical environment, of a sort that would enable it to live in the way it thought would bring it completeness, but which in fact, of course, does nothing of the kind. Now it is in the body of a human being, or perhaps a human male, and if it continues to misunderstand its own original nature—which is easier now for it to do, since it has come to feel at home in an actual physical existence, and to become habituated to various kinds of embodied pleasures—then it seeks to be in a body, and always a body that would best express the way it wants to live. So on physical death, a soul in this state is reincarnated, perhaps as another human being, but also perhaps (so Plato held, to the great embarrassment of some of his admirers) as a lower animal, say a pig or wolf whose wallowing or ravening life-style fleshes out the soul's most precious previous desires.6 Alternatively, the embodied soul may incline towards disembodiment, and achieve it or come closer to achieving it by practising its intellectuality and rejecting physical and worldly enthusiasms. This is why, in the Phaedo, about-to-die Socrates tries to comfort his friends by telling them that if death is the separation of soul from body, the philosopher should be glad to die, since the philosopher has lived his present life gladly practising for death by losing himself to intellectual activity.

In Plato, then, the question of separability of soul from body is not a simple one. In the first place, every embodied soul is separable from its current body, since the soul is immortal, whereas any given body will wear out. Secondly, every soul is in principle separable from body altogether, since every embodied soul is in principle, or at least by virtue of its original nature, able to refine itself to the point where it wants nothing that a body can provide. However, saying this is a bit like saying: human beings by nature can live without heroin or cocaine; heroin and cocaine addicts are human beings by nature; therefore they can live without heroin or cocaine. Granted they have the capacity, they lack the power to exercise it as of now, just as human beings by contrast with bull-frogs have the capacity to speak several different languages, but someone who has never learnt a foreign language lacks the ability to exercise this human capacity. In this sense, some embodied souls cannot live separate from a body suited to their desires, while others, a minority perhaps, can.

According to this picture, the body is simply the instrument of the soul, a view that Aristotle too would endorse at one stage of his career. That is, the soul does not depend on the body except to do through it something that it wants to do. Thus it fashions and animates its body for the sake of physical action, sensation and experience. That the soul can do this if it chooses goes along with the thought, which we find again and again in Plato, that the soul is divine or godlike. This means that in itself it has a sort of limited omnipotence. If it wills or really desires a certain kind of life for itself, its 'will is done' even if it wills what is bad for it: automatically it comes to be equipped with what is necessary. But once it is in a body, of course, what it can bring about is limited by the nature of its body and the environment.

So—to answer our earlier questions about the essence of soul—the soul for Plato is essentially a valuing power: a power to create and maintain for itself the life it truly desires and thinks good, along with that lifestyle's accoutrements or freedom from accoutrements. Its purely intellectual function and its body-animating function represent different bents or inclinations. If we consider soul in general and in the abstract, it is presumably contingent whether soul is embodied, and embodied this way or that, or whether it is pure intellect. What is essential and fundamental is soul's determinability, in fact self-determinability, in contrary ways. If, however, we consider an individual soul, its determinate condition—its being embodied or not, and if embodied then how—is all but fundamental for this individual. For on the one hand this condition reflects the individual's currently dearest values, and on the other hand it affects almost everything the individual does and experiences in its current life.

We may wonder how the soul is supposed to take on a body. Plato says little about this. At one point he seems to suggest that the soul 'weaves' a body for itself.7 Certainly he does not want to imply that the soul has hands and moves a shuttle to and fro. The idea presumably is that the soul informs certain materials which in its presence grow and organise themselves into the requisite body. A previously embodied soul may start with some matter from its previous body.8 Plato shows no sign of holding that the soul creates its body ex nihilo.

Some philosophers might balk at the idea that the soul has power to re-arrange matter. They might, if they accepted the existence of the soul at all, feel more comfortable with the thought that the soul actually dreams its body and physical environment. Some work would then have to be done to explain whether, and if so, how, souls dreaming different physical dreams nonetheless in some sense share a world with each other. But this is not Plato's problem, for he does not strike out in the idealist direction.

It is sometimes suggested that one needs to have been bitten by the bug of external-world scepticism before one can seriously consider idealism. Certainly the bug of external-world scepticism did not get to Plato. But there is something else one should bear in mind when considering Plato's silence on these great questions of modern philosophy. The fact is that from the point of view of Platonic ethical concern, which is a point of view that pervades most of the dialogues, it makes no difference whether the soul chooses to dream, and then becomes addicted to dreaming, its embodiment, or whether it chooses and then becomes addicted to life mediated by a real, independently existing, body in a real physical environment.9 Whereas for Descartes this makes all the difference—one way God is a deceiver, the other way not—for Plato either way the soul in question gets what it wants, and is just as misguided in wanting it if the body turns out to be independently real as it would be if the body were its fantasy.

I have been comparing Plato's argument in the Phaedo with Descartes' in the Meditations that soul is separable from body. Let me end by comparing some of the wider purposes of those arguments. Plato offers the argument of the Phaedo as, inter alia, an instance and example of the kind of intellectual exercise that loosens the human soul's attachment to its body. Since the attachment reflects the soul's misunderstanding of the true nature of happiness, the Phaedo argument, for those who enter into it, is an exercise in soul-saving. By contrast, what Descartes discovers when he discovers his reasons for declaring the mind separable from the body is entirely different from the intellectualization he himself undergoes in order to reach the proof. And he cannot overtly, even if he is inclined so inwardly, claim this refinement as a sort of soul-saving without running foul of the religion of his time. For although this religion differed within itself on how much faith counts for salvation, and how much works, these were the only options considered, and Descartes' activity does not come under either. Instead, his avowed purpose in following the path of the Meditations from doubt to himself, and from himself to the God who is not a deceiver, is to establish 'something firm and lasting in the sciences',10 i.e. mathematics and mathematical physics.

This is an extremely puzzling remark if it means that these sciences fail as sciences if they cannot be rendered indubitable by an argument that first doubts and then reinstates the clear and distinct ideas on which such inquiries depend. For the mathematician's performance as such is not less clear or less accurate if he lacks a proof to the effect that although the most rigorous mathematics conceivable to man can be doubted, nonetheless in the end we are theologically justified in accepting them. But surely Descartes' hope is not to make the mathematician a more successful mathematician, but rather to show the rest of us that mathematical science in its own sphere carries the same authority as divine revelation in its, since both come from the same source. Rightly understood, the practice of such abstract studies, though not a religious exercise, is not secular either, for it expresses God as reason or the natural light. Plato would surely have agreed that it is not secular, but he could not have imagined the historical context that made it so important for someone in Descartes' position to distinguish priest and mathematical scientist, in effect postulating at least two kinds of 'higher calling', one devoted to faith, the other to reason.


1. A version of this paper was delivered as the 2000 Foerster Lecture on the Immortality of the Soul, at the University of California at Berkeley.

2. In the Timaeus it is taken for granted that the world-soul must have a body; and purified human intellects return to spatial locations in stars.

3. For Descartes these include memories so far as the latter depend on images grounded in the body.

4. Thus phronein (= 'to have one's wits about one') ranges in meaning from 'to be sane' to 'to be conscious'.

5. If bare biological animation is thought of as continuous in kind with intelligent physical activity such as playing tennis or cooking, it will seem plausible that theoretical contemplation at its fullest depends on suspension of animation, since it seems to be a fact, and not a merely contingent one, that attention used in theoretical contemplation is attention taken away from intelligent physical activity, and vice versa. Plato models intellection on dreaming, which the soul is free to do only when not governing the limbs and perceiving through the sense organs in waking life (cf. the Hippocratic treatise On Dreams [Regimen IV], 86). However, Plato then turns things round with his familiar dictum that the waking world is that of the eternal intelligibles, the dream world that of everyday life.

6. Aristotle was unfair if he meant to include Plato in his criticism of the Pythagoreans for assigning 'any chance soul to any chance body' (De Anima I 3, 407b 20-24).

7.  Phaedo 87b-e. The weaving idea occurs as part of a view that is rejected, but what is rejected is not the weaving, but the thought that, as with an actual weaver, the soul might cease to exist before wearing out its final coat.

8.  Cf. Phaedo 80c-81c.

9.  Plato can of course make this distinction even if, as I am arguing, it does not carry for him a burning question; but the word 'real' used as above would presumably not be his tool for making it, since his realia are immutable Forms.

10.  Meditation I, first paragraph.


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