DESCARTES

I am starting from the assumption that the body is nothing other than a statue or machine made of earth, which God deliberately makes as similar to us as possible. Consequently, he not only gives it the external colour and shape of all the parts of our bodies, but also puts in all the components necessary to make it walk, eat, breathe, and in short imitate all those of our functions that can be imagined to come from matter and depend only on the disposition of our organs.

We see clocks, artificial fountains, mills and other similar machines that have the power to move themselves in many different ways, although they are made only by humans; and I am sure I could not even imagine as many sorts of movements in this bodily machine as I suppose have actually been made by the hands of God; and however much skill I might attribute to him, you would have reason to believe that his skill was even greater than I had said. . . . [He then describes the circulation of the blood, and says that the function of the heart is to warm it and distil off its ‘subtler’ parts.]

[128] But what must be principally noticed here is that the most lively, forceful and subtle parts of the blood flow into the cavities of the brain, since the arteries which lead there are the ones that come most directly of all from the heart, and, as you know, all moving bodies tend to continue their motion in a straight line as far as possible. . . .

[129] As for the parts of the blood which penetrate right into the brain, they serve not merely to nourish and maintain its substance, but their main function is to produce in it a certain extremely subtle wind, or rather an extremely lively and pure flame, called the ‘animal spirits.’ I should explain that the arteries which carry them from the heart divide into an infinity of little branches, which make up the little tissues spread out like carpets on the bottom of the cavities of the brain, and then come together round a certain little gland, situated about the middle of the substance of the brain right at the entrance of the cavities. At this point the arteries have a large number of little holes which let the subtlest parts of the blood flow into the gland, but which are too narrow for the grosser ones to pass through. . . .

[130] So, once the spirits have thus passed [via the pineal gland] into the cavities of the brain, they proceed from there into the pores of its substance, and from the pores into the nerves. Once there, depending on which nerves they have entered (or even only tended to enter) and in what amounts, they have the power to change the shape of the muscles the nerves end up in, and in this way to make all the parts of the body move. A perfect analogy would be the grottoes and fountains in the Royal Gardens, which you may have seen. In them, water power is by itself sufficient to set the various machines going, and even to make them play instruments or utter words, depending on the dispositions of the water pipes.

In fact the nerves of the machine I have described [131] are just like the pipes of the machinery of the fountains; its muscles and tendons are like the engines and springs use to move them; its animal spirits are like the water which drives them — the heart being the source, and the cavities of the brain the control pit. Further, respiration and other such natural and ordinary actions depending on the flow of spirits are like the movements of a clock or mill, which the ordinary flow of water can make continuous. External objects, which act on its sense organs by simple contact and thus determine it to move in many different ways depending on the disposition of the parts of its brain, are like visitors, who, on entering some of the fountain grottoes themselves unwittingly cause the movements which happen in their presence: for when they go in they cannot help treading on certain flagstones disposed in such a way that if they approach a bathing Diana, for example, they will make her hide among the reeds; and if they go further on to pursue her, they will make a Neptune approach them and threaten them with his trident; or if they go in some other direction they will make a sea monster come out and squirt water in their faces — or other such things according to the whims of the engineers who made them. And finally, when the Rational Soul is in the machine, it will have its principal seat in the brain, and there it will be like the person in charge of the fountains, who must be in the control pit (where all the pipes from the machines come together), when he wants to set them going, or stop them, [132] or change their movements in some way. . . .

[141] Next, I shall explain how this machine can be stimulated to move all its parts in a thousand other ways by the external objects which impinge on its sense organs. The little threads which, as I have already said, come from right inside its brain and form the marrow of its nerves, are so disposed in all of their parts which serve as the instruments of various senses, that they can easily be moved by the objects of theses senses; and when they are moved, however slightly, they simultaneously pull at the parts of the brain they come from, and thereby open the entrances of particular pores on the inner surface of the brain. In this way, the animal spirits in its cavities immediately begin to move and flow though the pores into the nerves and muscles. These processes in this machine result in movements just like those we are naturally incited to when our senses are stimulated in the same way. . . .

[143] But I will tell you that when God unites a rational soul to this machine (I shall explain how later), he will give it its principal seat in the brain, and make it of such a nature that it will have various sensations according to the various ways in which the entrances of the pores on the internal surface of the brain are opened by the agency of the nerves.

As a first example, let us suppose that the little threads which compose the marrow of the nerves are pulled with so much force that they break and come away from the part they are attached to, so that the structure [144] of the whole machine will thereby become in some sense less perfect. In this case, the movement they cause in the brain will give occasion to the soul to have the sensation of pain, since the preservation of its dwelling place is a matter of importance to it. . . .

[165] If you have ever had the curiosity to take a close look at a church organ, you will know how the bellows force the air into certain vessels (technically known as ‘wind-trunks’, I believe); and how the air goes from there into the pipes, in combinations depending on the various ways the organist moves their fingers over the keyboard. But here you could imagine that the heart and the arteries pushing the animal spirits into the cavities of the brain of our machine, are like the bellows of an organ forcing the air into the wind-trunks; and that the external objects making the spirits in the cavities enter specific pores, depending on which nerves they stimulate, are like the fingers of the organist making the air enter the wind-trunks of specific pipes, depending on which keys they depress. Again, the harmony of an organ depends neither on the arrangement of the pipes visible from the outside, nor on the shape of its wind-trunks or other parts, but only on three [166] things: namely, the air which comes from the bellows, the pipes which produce the sound, and the distribution of the air in the pipes. Similarly, I want to point out to you that the bodily functions we are dealing with here do not in any way depend on the external shape of all those visible parts which anatomists distinguish in the substance of the brain or its cavities; but only on the spirits which come from the heart, on the pores in the brain through which they pass, and on the way the sprits are distributed in these pores. Hence all that remains is for me to explain one by one what is most noteworthy in these three things. . . .

And notice that I say ‘imagines or senses’; for I want to include generally under the term ‘idea’ all the impressions the spirits can receive on leaving the gland H. As long as they depend on the presence of objects, they are all attributed to the common sense; but, as I shall say later, there are many other causes they can proceed from, and in such cases it is to the imagination that they must be attributed.

I might mention here how the traces of those ideas pass through the arteries towards the heart, and thus radiate throughout the blood; and how it is sometimes even possible for certain activities of a pregnant woman to make them get impressed on the parts of the body of the infant being formed in her womb. But for the present I shall content myself with telling you how ideas impress themselves on the interior part of the brain, marked B, which is the seat of the Memory. . . .

[179] In addition, you should observe that the gland H is composed of a very soft substance, and that it is not completely joined and united to the substance of the brain, but only attached by some little arteries (the walls of which are relatively slack and pliant) and supported in a sort of equilibrium by the force of the blood which the heat of the heart forces towards it. Consequently, it needs only the tiniest thing to disturb the balance and make it lean in one direction or another to a greater or lesser degree, and thereby make it dispose the spirits coming out of it to make their way towards certain parts of the brain rather than others.

[180] Now there are two principal causes (apart from the force of the soul, which I shall bring up later) that can make it move in this way, and I must now explain these to you.

The first is the differences there are between the small parts of the spirits coming out of it. . . .

[185] But in my opinion, the most noteworthy consequence of memory [as a natural faculty of the brain] consists in the fact that this machine, without the presence of any soul and simply by virtue of the dispositions natural to it, is capable of imitating all the movements which real people, or even other similar machines, will make with a soul present.

The second cause which can determine the movements of the gland H is the action of objects affecting the senses. . . .

[202] I want you to take note that in this machine all these functions follow naturally from the disposition of its organs alone, no more and no less than those of a clock or automaton follow from that of its counterweights and wheels. Consequently they give us no reason at all for postulating in it any other vegetative or sensitive soul, or any other such principle of movement or life, than its blood and spirits, activated by the warmth of the fire which continuously burns in its heart and which is of precisely the same nature as all the fires to be found in inanimate bodies. [The treatise breaks off at this point.]