Descartes – Mind and Brain



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Descartes – Mind and Brain



The Scholastic Picture: Perception and Intellection
Simple creatures such as plants differ from inanimate objects by the possession of a vegetative soul in virtue of which they grow and reproduce. Animals have a vegetative soul and a sensitive soul, as they can sense the world. Humans uniquely have an intellectual soul as well, in virtue of which they can think and will conceptualised thoughts.
We perceive the world by the forms of things entering the appropriate sensory organ. The forms that enter my eye (e.g.) are however not exactly the same in kind as those in things. The redness of the tomato does not move through the air into my eye – otherwise the air and my eye would be red. So, we have to say that corresponding to the formal form of redness there is the intelligible or intentional form of redness. It is this type of form that passes from the object to my eye. The intentional or intelligible forms received through the sense organs end up in the ‘common sense’ or sensorium which integrates them in a unified perceptual experience of the world. My intellect is then able to act on the common-sense. So, in judging that there is a tomato before me, my intellect picks up on the data in the sensorium and activates the concept tomato.
Descartes thought this was all rubbish. He disparaged the idea of “all those little images flitting through the air, called ‘intentional forms’, which so exercise the imagination of the philosophers.” (Optics. CSM I 153; AT VI 85). Once again, the claim is that this is no real explanation. To say that I know that there is a badger before me because the form of the badger – okay, the intentional form of the badger – was received by my sense organs and then received from the sensorium by my intellect which found it to fall under the concept badger does not enlighten me.
Descartes’ View: The Brain
Descartes is known as an interactionist dualist. We are a combination of mind and body. The popular view is that they interact at the pineal gland. This is not quite right, for reasons we shall see.
Let’s start with the body. It’s a machine, as Descartes makes clear:
And truly one can well compare the nerves of the machine that I am describing to the tubes of the mechanisms of these fountains, its muscles and tendons to divers other engines and springs which serve to move these mechanisms, its animal spirits to the water which drives them, of which the heart is the source and the brain's cavities the water main. Moreover breathing and other such actions which are ordinary and natural to it, and which depend on the flow of the spirits, are like the movements of the clock or the mill which the ordinary flow of the water can render continuous. External objects which merely by their presence act on the organs of sense and by this means force them to move in several different ways, depending on how the different parts of the brain are arranged, are like strangers who, on entering some of the grottoes of these fountains, unwittingly cause the movements that then occur, since they cannot enter without stepping on certain tiles which are so arranged that, for example, if they approach a Diana bathing they will cause her to hide in the reeds; and if they pass farther to pursue her they will cause a Neptune to advance and menace them with his trident; or if they go in another direction they will make a marine monster come out and spew water into their faces, or other such things according to the whims of the engineers who made them. And finally when there shall be a rational soul in this machine, it will have its chief seat in the brain and will reside there like the turncock who must be in the main to which all the tubes of these machines repair when he wishes to excite, prevent, or in some manner alter their movements. (Treatise on Man: CSM I 100-1; AT XI 131-2).
Similarly, our brain is a physical thing. It is a machine and to be explained in mechanical terms. We are machines, bodily speaking, just like animals.
Light enters our eyes, triggers motion in the optic nerve. The sensation is then a pattern in my brain. Indeed, we can – if we are careful – call it an idea.
But of these figures [i.e. patterns of activity in the animal spirits], it is not those that are impressed on the organs of the external senses or on the internal surface of the brain, but only those traced in the spirits on the surface of the gland H [i.e. the pineal gland] which is the seat of the imagination and of the common sense, that are to be taken as being the ideas (i.e. the forms or images) which the rational soul will consider directly when, once it is united to this machine, it imagines or senses some object. (Treatise on Man: CSM I 106, AT XI 176-7)
It is the pattern on the pineal gland that is an idea as, in being on the pineal gland, my mind is able to become aware of it.12 Now, Descartes is at pains to say that an idea of X does not resemble its object X in a simple, pictorial way:
[There] need be no resemblance between the ideas which the soul conceives and the movements [of the spirits] which cause these ideas. You will readily grant this if you note that people struck in the eye seem to see countless sparks and flashes before them, even though they shut their eyes or are in a very dark place; hence this sensation can be ascribed only to the force of the blow, which sets the optic nerve-fibres in motion as a bright light would do. (Optics: CSM I 168-9; AT VI 131)
For this leads to the homuncular fallacy. If there really is a picture-like idea in my head, then we need to posit an “inner eye” to perceive it. But if this inner eye works by having its own little ideas that are picture-like copies of the idea, then we need to posit an inner-inner eye to perceive them…and so on. The problem arises because we are not bothering to explain how an idea represents. We are simply saying that ideas just look like what they are ideas of and hoping that is enough. It is not. Descartes is to be credited for realising that representation does not require resemblance. The word “badger” represents the animal and clearly looks or sounds nothing like a badger. There is a conventional relation of representation here. Similarly, so long as the pattern on the pineal gland stands in such a relation, so that (e.g.) seen badgers produce pattern X on the gland, that’s enough.
If I perceive, animal spirits leave a trace on the pineal gland. If I store the image, they modify the structure of my brain. If I recall the image, I reactive the pattern in the brain. If imagine something, my brain is caused to produce patterns as if it were receiving data from the senses.
Descartes’ View: Perception
Thoughts can be active (from the will) or passive. Passive thoughts are perceptions. In perceiving, I have a sensation. We can now divide perceptions as follows:


  • To perceive something in the “ordinary sense” is to have a sensation which I refer correctly to something outside of my body.

    • To hallucinate or dream is refer incorrectly to something outside of my body.

  • To sense genuine hunger is to have a sensation I refer correctly to something inside my body.

    • If I can mistakenly have a sensation of hunger, perhaps when I am ill and eating would harm me, then this is mistaken in the same way a perception is.

  • To experience an emotion or sensation is to have a sensation I refer to the soul.

    • You can’t have a mistaken emotion. You can’t seem to be angry: you are or you aren’t. (Note that you can of course be mistaken about the reason for your anger or object of your anger – that’s different.)


Descartes’ View: Emotion
Finally, to have an emotion or sentiment is to perceive a state of one’s body but in a way such that I do not refer it to the body. Consider anger. When I am angry, my heart-rate goes up. This bodily state of affairs is relayed to my brain by the animal spirits and then to the pineal gland. In being angry, I do not automatically refer that sensation to something external to me (as I may not be able to identify what I am angry about) or to my body (I don’t think of anger as an internal state like hunger.)
What is the point of the emotions? In the Passions of the Soul, Descartes writes:
§40 The principle effect of the passions [=emotions/sentiments] [is to] move and dispose the soul to want the things for which they prepare the body. Thus the feeling of fear moves the soul to want to flee, that of courage to want to fight, and similarly with the others.
Let’s work through a case to see how all this fits together.


  1. A tiger enters the room.

  2. I sense the tiger: there is a pattern in my brain.

  3. This triggers a memory of tigers being dangerous.

  4. This triggers raised heart-rate and blood pressure as I need to get ready to run.

  5. My brain is aware of this physiological state of my body thanks to nerves ‘reporting back’ and hence realising a distinctive pattern of brain activity.

  6. The brain-pattern representing the tiger derived from the eyes and the brain-pattern of the state of my body are impressed on the pineal gland. I thus perceive the tiger and experience the fear.

  7. The job of the emotion is to tell my mind that the running-away my body is preparing for is something that I should want. (Because being eaten is a bad thing.)

So, the pineal gland receives impressions – brain activity in the form of animal spirits – which make it possible to have conscious states of awareness and thoughts about what I perceive along with experiences of bodily sensations such as pain and fear, where the difference between pain and fear is that in the former case, I know that the sensation is purely bodily whereas in the latter I don’t.


Going the other way, should I want my thoughts to affect the world, my mind must agitate the pineal gland in such a way as to move the spirits so that they cause movement in my body of the appropriate kind.
Descartes’ View: The Mind and Interactionism
An animal senses just like we do. It has no mind. So, sensation is just a feature of the brain. At the same time, as minded creatures, we make intellectual judgments about our sensations. We see a tiger-shape as a tiger, for example. What happens here is that the animal sprits in the brain act on up on the pineal gland and in so doing cause my mind to be involved and to make judgments about the world I perceive.
So, conscious perception occurs because we have a mind interacting with the brain via the pineal gland. Yet Descartes is clear that he sees intellection, will, sensation and memory one thing. The mind is not literally divided into several faculties that reside in different bodily organs. This is why it is not quite right to say that mind and brain interact via the pineal gland – it makes them sound too much as if they are separate but co-operating entities. Of course, this raises the big problem: since my mind is not identical with my brain and my brain is involved in sensation, etc. how can we speak of one thing at work? This is just the problem of understanding Descartes’ interactionism.
It is very important to note, however, just how far Descartes is going to involve the brain. Descartes is not saying that there’s a non-physical mind that handles everything: sensing, thinking, emoting, and so on. The brain – and body – are heavily involved. This is grounded in good science. Our senses are in our bodies. Emotions are felt in the body. They are also importantly isolated from the mind. Try as I might, I cannot stop myself seeing the tiger that is in front of me by wanting my visual sensations to be different. Try as I might, I cannot stop myself being afraid just by my will issuing a command. Descartes rightly sees that if I am to alter my emotional responses, I must go about it indirectly. Let us suppose I can will myself to run at the tiger. I find that the tiger is surprised and becomes tame. I am still experiencing the fear but it diminishes with my discovery. I face other tigers and find that they also become tame. I learn that they are nothing to be afraid of. These memories alter my brain such that when I see tigers in future, my body does not go into the physiological state of getting-ready-to-run, the perception of which is fear.
Similarly, you’ll recall from Meditation VI how Descartes says that we cannot imagine a chiliagon but we can think about it. Imagination requires a bodily effort whereas intellection does not.
Finally, Descartes, in his correspondence with Elisabeth, discusses how bodily illness can affect our capacity to think.
Man, Machine and Language
A man is a machine but not just a machine as he has a mind. It is in virtue of having a mind that we have the capacity to use language and not to be limited in our range of actions, says Descartes. The common point is creativity. Prefiguring a complaint Turing raises against himself some three centuries later, being a point very commonly made against the possibility of intelligent robots, the thought is that any system whose behaviour, linguistic or otherwise, is rule-governed, will be limited to a fixed repertoire of responses whereas humans are able to respond in novel and appropriate ways to an indefinitely wide range of situations:
"Such persons will look upon this body as a machine made by the hands of God, which is incomparably better arranged, and adequate to movements more admirable than any machine of human invention. Were there such machines exactly resembling in organs and outward form an ape or any other irrational animal, we could have no means of knowing that they were in any respect of a different nature from these animals; but if there were machines bearing the image of our bodies, and capable of imitating our actions as far as it is morally possible, there would still remain two most certain tests whereby to know that they were not therefore really men. Of these the first is that they could never use words or other signs arranged in such a manner as is competent to us in order to declare our thoughts to others. The second test is, that although such machines might execute many things with equal or perhaps greater perfection than any of us, they would, without doubt, fail in certain others from which it would be discovered that they did not act from knowledge, but solely from the disposition of their organs. Again, by means of these two tests we may know the difference between men and brutes.” (Discourse on the Method pt. V: CSM I 139-40; AT VI 56-58)



1 I have made it sound as if animal spirits flow to the brain and leave an imprint on the pineal gland. In fact, it is the other way around. The pineal gland is constantly emitting animal spirits. Here’s a clear explanation from a good article on the pineal gland:
“In Descartes' description of the role of the pineal gland, the pattern in which the animal spirits flow from the pineal gland was the crucial notion. He explained perception as follows. The nerves are hollow tubes filled with animal spirits. They also contain certain small fibers or threads which stretch from one end to the other. These fibers connect the sense organs with certain small valves in the walls of the ventricles of the brain. When the sensory organs are stimulated, parts of them are set in motion. These parts then begin to pull on the small fibers in the nerves, with the result that the valves with which these fibers are connected are pulled open, some of the animal spirits in the pressurized ventricles of the brain escape, and (because nature abhors a vacuum) a low-pressure image of the sensory stimulus appears on the surface of the pineal gland. It is this image which then “causes sensory perception” of whiteness, tickling, pain, and so on.” (http://plato.stanford.edu/entries/pineal-gland/)


2 Why the pineal gland? In a letter to Mersenne, Descartes writes:
My view is that this gland is the principal seat of the soul, and the place in which all our thoughts are formed. The reason I believe this is that I cannot find any part of the brain, except this, which is not double. Since we see only one thing with two eyes, and hear only one voice with two ears, and in short have never more than one thought at a time, it must necessarily be the case that the impressions which enter by the two eyes or by the two ears, and so on, unite with each other in some part of the body before being considered by the soul. Now it is impossible to find any such place in the whole head except this gland; moreover it is situated in the most suitable possible place for this purpose, in the middle of all the concavities; and it is supported and surrounded by the little branches of the carotid arteries which bring the spirits into the brain” (29 January 1640, AT III:19-20, CSMK 143).
Historically, the pineal gland had been considered as having this role. However, it was considered important – or, rather, it or something like it. Let me explain. Certain philosophers thought that mental activity took place in the ventricular system of the brain – a system of cavities filled with cerebrospinal fluid. Nemesius of Emesa (ca. 400 AD) claimed that three different ventricles were the seats of imagination, reason and memory. He thought, as later philosophers did, that they were filled not with fluid but with animal spirits. It turns out that there is, amidst this system, a little “worm-like” entity (called today the vermis superior cerebelli) that was regarded as a device that controlled the flow of animal spirits. So, the Arab philosopher Qusta Ibn Luqa argued that memories were in the posterior ventricle and that people look up when they want to remember because this raises the worm-like entity and allows the memories to “flow out”. In the picture on the right below, you can see the “worm” represented (it looks like a little lizard). (The history here and much more can be found at http://plato.stanford.edu/entries/pineal-gland/ )



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