Department of History
University of California, Irvine
 Instructor:    Dr. Barbara J. Becker


Week 4.  Sensation

 excerpts from
Treatise on Man (1632-c.1640)
by René Descartes (1596-1650)
trans. (1998) Stephen Gaukroger

[Part 1:   On the machine of the body]

...I suppose the body to be just a statue or a machine made of earth, which God forms with the explicit intention of making it as much as possible like us.  Thus He not only gives its exterior the colours and shapes of all the parts of our body, but also places inside it all the parts needed to make it walk, eat, breathe, and imitate all those functions we have which can be imagined to proceed from matter and to depend solely on the disposition of our organs.

We see clocks, artificial fountains, mills, and other similar machines which, even though they are only made by men, have the power to move of their own accord in various ways.  And, as I am supposing that this machine is made by God, I think you will agree that it is capable of a greater variety of movements than I could possibly imagine in it, and that it exhibits a greater ingenuity than I could possibly ascribe to it....

First, food is digested in the stomach of this machine by the force of certain fluids which, gliding among its parts, separate, shake, and heat them, just as ordinary water does those of quicklime, or aqua fortis those of metals....

[T]hese particles ... descend gradually toward the passage through which the coarsest of them must exit.  And the finest and most agitated meanwhile encounter innumerable small holes through which they flow into the branches of a large vein that bears them toward the liver, and into others that bear them elsewhere, with nothing but the small size of the holes serving to separate these from the coarser particles; just as, when one shakes meal in a sieve, the purest parts flow out and it is only the small size of the holes through which it passes that prevents the bran from following after them....

[T]he pores of the liver are arranged in such a way that this fluid, on entering, is refined and transformed, taking on the colour and form of blood....

Now there is only one passage evident by which this blood, thus contained in the veins, can leave them, namely that which conveys it to the right cavity of the heart.  And note that the flesh of the heart contains in its pores one of those fires without light which I have spoken about earlier and which makes it so fiery and hot that, to the extent that the blood enters either of its two chambers or cavities, it is promptly inflated and expanded....  And the fire in the heart of this machine that I am describing to you has as its sole purpose to expand, warm, and refine the blood that falls continually a drop at a time through the passage from the vena cava into the cavity on its right side, from where it is exhaled into the lung, and from the vein of the lung which anatomists have called the 'venous artery' into its other cavity, from where it is distributed throughout the body....

[R]espiration['s] sole purpose in this machine is to thicken the vapours, as is necessary for maintaining the fire in its heart as it is in us for maintenance of our life....

The pulse, or beating of the arteries, depends on eleven small membranes which like so many small doors, close and open the orifices of the four vessels that open into the two cavities of the heart.  For at the moment when a beat ceases and another one is ready to begin, the small doors at the orifices of the arteries are shut tight, while those at the orifices of the two veins are open, so that two drops of blood cannot but fall immediately from these two veins, one into each cavity of the heart.  These drops of blood, being rarefied and suddenly occupying a space which is incomparably greater than that which they occupied previously, then push the small doors at the orifices of the veins shut, thereby preventing more blood from dropping into the heart, and they push open the arteries passing through them quickly and forcefully, and cause the heart and all the body's arteries to inflate at the same time....

[A]ll the most energetic, strongest, and finest parts of this blood proceed to the cavities of the brain, inasmuch as the arteries bearing them there are in the most direct line from the heart; and as you know, all moving bodies tend as much as they are able to continue their motion in a straight line....

[T]hose parts of the blood that penetrate as far as the brain, they serve not only to nourish and sustain its substance, but above all to produce there a certain very fine wind, or rather a very lively and very pure flame, which is called the 'animal spirits'....

[T]he coarsest parts of the blood can lose a lot of their agitation in the twists and turns of the little tissues through which they pass, to the extent that they have the power to push the smaller ones among them and so transfer some of their motion to them; but these smaller ones cannot lose their motion in this way , because the agitation is increased by that which the larger ones transfer to them and because there are no other bodies around them to which they can transfer theirs with the same ease.

[W]hen the coarsest parts go up straight to the external surface of the brain, where they serve to provide nourishment for its substance, they make the smallest and most agitated parts move out of the way, causing all of them to enter this gland, which we must imagine as a very full-flowing spring, and from this they flow at the same time and in every direction into the cavities of the brain.  And so, without any preparation or alteration, except being separated from the larger parts and retaining the extreme speed that the heat of the heart has given them, they cease to have the form of blood and are called animal spirits.

[Part 2:  How the machine of the body is moved]

Now as these spirits enter the cavities of the brain, they also pass in the same proportions from there into the pores of its substance, and from these pores into the nerves.  And depending on which of these nerves they enter, or even merely tend to enter, in varying amounts, they have the power to change the shapes of the muscles into which these nerves are embedded, and in this way to move all the limbs.  Similarly, you may have observed in the grottoes and fountains in the royal gardens that the force that drives the water from its source is all that is needed to move various machines, and even to make them play certain instruments or pronounce certain words, depending on the particular arrangements of the pipes through which the water is conducted.

And the nerves of the machine that I am describing can indeed be compared to the pipes in the mechanical parts of these fountains, its muscles and tendons to various other engines and springs which serve to work these mechanical parts, its animal spirits to the water that drives them, the heart with the source of the water, and the brain's cavities with the apertures.  Moreover, respiration and similar actions which are normal and natural to this machine, and which depend on the flow of spirits, are like the movements of a clock or mill, which the flow of spirits, are like the movements of a clock or mill, which the normal flow of water can make continuous  External objects, which by their mere presence act on the organs of sense and thereby cause them to move in many different ways, depending on the arrangement of the parts of the brain, are like strangers who on entering the grottoes of these fountains unwittingly cause the movements that take place before their eyes.  For they cannot enter without stepping on certain tiles which are arranged in such way ... depending on the whim of the engineers who constructed them.  And finally, when a rational soul is present in this machine it will have its principal seat in the brain and will reside there like the fountaineer, who must be stationed at the tanks to which the fountain's pipes return if he wants to initiate, impede, or in some way alter their movements....

[A] nerve['s] external membrane is like a large tube which contains several other tiny tubes ... made up from a finer internal membrane....

[Each] tiny nerve ... proceeds to muscle ... composed of a relaxed membrane which can be extended, enlarged, and shrunk depending on the amount of animal spirits that enter or leave it; and its branches or fibres are arranged in such a way that when animal spirits enter there they may cause the whole body of the muscle to inflate and shorten and so pull [that] to which it is attached, whereas when they withdraw, on the other hand, the muscle deflates and elongates again....

For you will readily recognise that these spirits, being like a wind or a very fine flame, must flow promptly from one muscle to another as soon as they find a passage, even though they are propelled by no other power than the inclination that they have to continue their motion in accord with the laws of nature.  And you will also recognise that even though they are very mobile and very fine, they have the strength to inflate and tighten the muscles which enclose them, just as the air in a balloon hardens it and stretches the membranes that enclose it.

Now you can easily ... understand how the machine that I am describing to you can be moved in all the ways that our body can, just by the force of the animal spirits that flow from the brain into the nerves....

Next, in order to understand how the external objects that strike the sense organs can instigate the machine to move its members in a thousand different ways, note that the tiny fibres ... are arranged in every part serving as the organ of some sense in such a way that they are easily moved by the objects of that sense.  And when they are moved, with however little force, they simultaneously pull on the parts of the brain from which they come and thereby open the entrances to certain pores in the internal surface of the brain.  The animals spirits in the cavities of the brain immediately begin to make their way through these pores into the nerves and so into the muscles, which act so as to cause movements in the machine very like those we are naturally instigated to make when our senses are similarly affected....

[Part 3:   The external senses of this machine and how they are related to ours]

...And note that although the fibres I speak of are slight, for all that they extend securely all the way from the brain to the parts that are farthest away, and there is nothing in between that breaks them or which, because of pressure, hinders their activity.  This is because, even though their parts are bent in countless ways, the tubes containing these fibres also carry the animal spirits to the muscles and these spirits, which always inflate the tubes to some degree, protecting the fibres from getting squashed and keeping them as taut as possible along the route from the brain, where they originate, to the places where they terminate.

Now I hold that when God unites a rational soul to this machine ... He will place its principal seat in the brain and will make its nature such that the soul will have different sensations depending on the different ways in which the nerves open the entrances to the pores in the internal surface of the brain.

Suppose for example that the tiny fibres that make up the marrow of the nerves are pulled with such a force that they are broken and separated from that part of the body to which they were attached, with the result that the structure of the machine is in some way less intact.  The movement that they will then cause in the brain, whose location must remain the same, will cause the soul to have the sensation of pain.

And if they are pulled by a force almost as great as this, but nevertheless are not broken or separated from the parts to which they are attached, they will cause a movement in the brain which, testifying to the good condition of the other parts, will cause the soul to feel a certain bodily pleasure which we call 'tingling'.  And this, as you may observe, is very similar to pain as regards its cause, but quite opposite in its effect.

But if many of these tiny fibres are pulled with equal force and all together, they will cause the soul to perceive that the surface of the body touching the surface of the limb where they terminate is smooth; and if the fibres are pulled with unequal force they will cause the soul to feel it is uneven and rough.

And if they are set in motion only slightly, and separately from one another, as they are constantly by the heat that the heart transmits to other bodily parts, the soul will have no more sensation of this than of any other normal bodily function.  But if this movement is increased or lessened by some unusual cause, its increase will cause the soul to have a sensation of heat, and its decrease a sensation of cold.  Finally, depending on the various other ways in which they are stimulated, the fibres will cause it to perceive all the other qualities that come under touch in general, such as humidity, dryness, weight, and so on.

It must be noted, however, that slight and mobile as these fibres might be, they are not sufficiently so as to be able to transmit to the brain all of the most subtle actions in nature.  In fact the slightest motions that they transmit are those involving the coarser parts of terrestrious bodies.  And even among those bodies there may be some whose parts, although rather coarse, can slide against the fibres so slightly that, even though they press against them or even cut through them completely, either action fails to be transmitted to the brain:  just as there are certain drugs that have the power to numb or even destroy the parts to which they are applied, without causing us to have any sensation of them at all.

[Part 4:  On the internal senses which are to be found in this machine]

...If you have ever had the curiosity to look closely at the organs in our churches, you will know how the bellows push the air into certain receptacles, which for this reason are named wind chests; and also how this air passes from there into one or another of the pipes, according to the different ways in which the organist moves his fingers on the keyboard.  You can think of our machine's heart and arteries, which push the animal spirits into the cavities of the brain, as being like the bellows of an organ, which push air into the wind chests; and of external objects, which displace certain nerves, causing spirits from the brain cavities to enter certain pores, as being like the fingers of the organist, which press certain keys and cause the wind to pass from the wind chests into certain pipes.  And just as the harmony of organs depends not on the externally visible arrangement of pipes or on the shape of the wind chests or other parts but solely on three factors, namely the air that comes from the bellows, the pipes that make the sound, and the distribution of air in the pipes; so too, I would point out, the functions that we are concerned with here do not depend at all on the external shape of the visible parts which the anatomists distinguish in the substance of the brain and in its cavities, but solely on three factors, namely, the spirits that come from the heart, the pores of the brain through which they pass, and the way in which the spirits are distributed in these pores....

[Part 5:  On the structure of the brain of this machine, and how the spirits are distributed there so as to cause its movements and its sensations]

...I desire that you consider that all the functions that I have attributed to this machine, such as the digestion of food, the beating of the heart and the arteries, the nourishment and growth of the bodily parts, respiration, waking and sleeping; the reception of light, sounds odours, smells, heat, and other such qualities by the external sense organs; the impression of the ideas of them in the organ of common sense and the imagination, the retention or imprint of these ideas in the memory; the internal movements of the appetites and the passions; and finally the external movements of all the bodily parts that so aptly follow both the actions of objects presented to the senses, and the passions and impressions that are encountered in memory:  and in this they imitate as perfectly as is possible the movements of real men.  I desire, I say, that you should consider that these functions follow in this machine simply from the disposition of the organs as wholly naturally as the movements of a clock or other automaton follow from the disposition of its counterweights and wheels.  To explain these functions, then, it is not necessary to conceive of any vegetative or sensitive soul, or any other principle of movement or life, other than its blood and its spirits which are agitated by the heat of the fire that burns continuously in its heart, and which is of the same nature as those fires that occur in inanimate bodies.

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  • Philosophical Letters between Mr. [John] Ray (1628-1705) and several of his Ingenious Correspondents.... (1718)
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