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


Week 4.  Sensation

 excerpts from
The Description of the Human Body and All Its Functions.... (1647-8)
by René Descartes (1596-1650)
trans. (1998) Stephen Gaukroger

[Part 1. Preface]

...[B]ecause it is the experience of everyone from childhood that many of our movements obey the will, which is one of the powers of the soul, this has disposed us to believe that the soul is the principle behind all of them.  And the ignorance of anatomy and mechanics has contributed to this, for in considering only the exterior of the human body, we never imagined that it had enough organs or springs in it to move itself in all the different ways in which we see it move.  And we have been confirmed in this error in judging that dead bodies have the same organs as living ones, for they lack nothing but the soul and yet there is no movement in them.

When we make the attempt to understand our nature more distinctly, however, we can see that our soul, in so far as it is a substance distinct from body, is known to us solely from the fact that it thinks, that is to say, understands, wills, imagines, remembers, and senses, because all these functions are kinds of thoughts....

[A]ll those movements that we do not experience as depending on our thought must not be attributed to the soul but only to the disposition of our organs; and even those movements that are called 'voluntary' proceed principally from this disposition of the organs, for they cannot have been produced without it, no matter how much we will it, and even though it is the soul that determines them.

Furthermore, although all these movements cease in the body when it dies and the soul leaves it, it should not be inferred from this that it is the soul that produces them, but only that the body's no longer being able to produce them and the soul's leaving it are due to the same cause....

...I shall try to ... explain the entire machine of our body in such a way that we will have no more reason to think that it is our soul that excites in us those movements that we do not experience as being directed by our will, than we have to judge that there is a soul in a clock that makes it tell the time....

First, I want the reader to have a general conception of all of the machine that I shall be describing.  I shall say here that the heat that it has in its heart is like the great spring or principle of all its movements, and that the veins are the tubes which conduct the blood from all the parts of the body towards the heart, where it fuels the heat there; just as the stomach and the intestines are another much larger tube, perforated with many little holes, through which the juices from the food run through the veins, which carry them straight to the heart.  And the arteries are yet another set of tubes, through which the blood, heated and rarefied in the heart, passes from there into all the other parts of the body, to which it brings heat and matter to sustain them.  Finally, the most agitated and most active parts of this blood are carried to the brain by arteries which follow the straightest line in their passage from the heart, comprising an air or very fine wind which is called the 'animal spirits'.  These dilate the brain, enabling it to receive both the impressions from external objects, and those from the soul, thereby acting as the organ or the seat of the common sense, of the imagination, and of the memory.  Then, this same air or these same spirits flow from the brain through the nerves into all the muscles, thereby making these nerves serve as organs of the external senses, and inflate the muscles in various ways imparting movement to all bodily parts....

[Part 4.  The bodily parts that are formed in the seed]

A still more perfect knowledge of how all the parts of the body are nourished is to be had when we consider how they were originally formed from the seed.  Until now, I have been unwilling to put my views on this topic into writing, because I have not yet been able to make enough observations to test all the thoughts I have had on the matter.  Nevertheless, I cannot refrain from setting out some very general points in passing, as I hope that these are those least likely to be among those that I will have to retract later, when new observations have enlightened me further.

I specify nothing concerning the shape and the arrangement of the particles of the seed:  it is enough for me to say that that of plants, being hard and solid, can have its parts arranged and situated in a particular way which cannot be altered without making them useless.  But the situation in the case of seed in animals and humans is quite different, for this is quite fluid and is usually produced in the copulation between the two sexes, being, it seems, an unorganised mixture of two liquids, which act on each other like a kind of yeast, heating one another so that some of the particles acquire the same degree of agitation as fire, expanding and pressing on the others, and in this way putting them gradually into the state required for the formation of parts of the body.

And these two liquids need not be very different from one another for this purpose.  For, just as we can observe how old dough can make new dough swell, and how the scum formed on beer is able to serve as yeast for making more beer, so we can easily agree that the seeds of the two sexes, when mixed together, serve as yeast to one another.

Now I believe that the first thing that happens in this mixture of seed, and which makes all the drops cease to resemble one another, is that the heat generated there -- which acts in the same way as does new wine when it ferments, or as hay which is stored before it is dry -- causes some of the particles to collect in a part of the space containing them, and then makes them expand, pressing against the others.  This is how the heart begins to be formed.

Then, because these tiny parts, which have been thus expanded, tend to continue in their movement in a straight line, and the heart, which has now begun to form, resists them, they slowly move away and make their way to the area where the brain stem will later be formed, in the process displacing others which move around in a circle to occupy the place vacated by them in the heart.  After the brief time needed for them to collect in the heart, these in turn expand and move away, following the same path as the former.  This results in some of the former group which are still in the same position -- together with others that have moved in from elsewhere to take the place of those that have left in the meantime -- moving into the heart.  And it is in this expansion, which occurs thus in a repeated way, that the beating of the heart, or pulse, consists.

But it should be noted, in connection with this material that passes into the heart, that the violent agitation of the heat that makes it expand not only causes some of the particles to move apart and become separated, but also some others to gather, pressing and bumping against one another and dividing into many extremely tiny branches which remain so close to one another that only the finest matter (which I called the 'first element' in the Principles of Philosophy) can occupy the spaces remaining around them.  And the particles that, in leaving the heart, join together with one another in this way, never leave the circuit by which they return to it, in contrast to the many other particles that penetrate the mass of seed more easily, and from the seed new particles continue to move towards the heart, until it is all used up.

And this -- as those who know my explanation of the nature of light in my Dioptrics and Principles of Philosophy, and the nature of colours in my Meteors, will easily understand -- is why the blood of all animals is red.  For I showed there that what makes us see light is simply the pressure exerted by matter of the second element, which I explained was made up of many little corpuscles all touching one another; and that we can observe two motions in these corpuscles:  one, that by which they follow a straight line towards our eyes, which gives us the sensation of light; the other, that by which, at the same time, they turn about their own centres.  And if the speed at which they turn is much less than that of their rectilinear motion, the body from which they come appears blue, whereas if they turn very much more quickly, it looks red to us.  But the only kind of body that could make them turn faster is one whose tiny parts have branches so delicate and so close to one another that the only matter turning around them is that of the first element, and I have shown blood to be like this.  The little corpuscles of the second element encounter, on the surface of the blood, this first-element matter, which continually passes with a very rapid oblique motion from one of these pores to the next, thus moving in the opposite direction to the corpuscles, and they are forced by this first-element matter to turn around their centres, and even to turn more with a greater speed than could be caused in any other way, since the first element surpasses all others in speed.

It is for much the same reason that iron, when it is hot, and coals, when they are burning, appear red:  for then many of their pores are filled only with the first element.  But because these pores are not as small as those of blood, the shade of red is different from that of blood....

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