Delphi Collected Works of René Descartes

by René Descartes

  Now, as regards heat, the sensation that we have of it can, it seems to me, be taken for a type of pain when it is violent, and sometimes for a type of tickling when it is moderate.11 Since we have already said that there is nothing outside our thought that is similar to the ideas we conceive of tickling and pain, we can well believe also that there is nothing that is similar to that which we conceive of heat; rather, anything that can move the small parts of our hands, or of any other part of our body, can arouse this sensation in us. Indeed, many experiences favor this opinion. For merely by rubbing our hands together we heat them, and any other body can also be heated without being placed close to a fire, provided only that it is shaken and rubbed in such a way that many of its small parts are moved and can move with them those of our hands.

  As regards light, one can also well imagine that the same motion that is in the flame suffices to cause us to sense light. But, because it is in this that the main part of my design consists, I want to try to explain it at some length and to take up my discourse from anew.

  CHAPTER THREE On Hardness and Liquidity

  I consider that there is an infinity of diverse motions that endure perpetually in the world. After having noted the greatest of these (i.e. those that bring about the days, months, and years), I take note that the vapors of the earth never cease to rise to the clouds and to descend from them, that the air is forever agitated by the winds, that the sea is never at rest, that springs and rivers flow ceaselessly, that the strongest buildings finally fall into decay, that plants and animals are always either growing or decaying; in short, that there is nothing anywhere that is not changing. Whence I know clearly that it is not in the flame alone that there are a number of small parts never ceasing to move, but that there are also such parts in every other body, even though their actions are not as violent and they cannot, due to their smallness, be perceived by any of our senses.

  I do not stop to seek the cause of their motion, for it is enough for me to think that they began to move as soon as the world began to exist. And that being the case, I find by my reasoning that it is impossible that their motions should ever cease or even that those motions should change in any way other than with regard to the subject in which they are present. That is to say, the virtue or power in a body to move itself can well pass wholly or partially to another body and thus no longer be in the first; but it cannot no longer exist in the world. My arguments, I say, are enough to satisfy me above, but I have not yet had occasion to relate them to you. In the meantime, you can imagine if you choose, as do most of the learned, that there is some first mover which, rolling about the world at an incomprehensible speed, is the origin and source of all the other motions found therein.

  Now, in consequence of this consideration, there is a way of explaining the cause of all the changes that take place in the world and of all the variety that appears on the earth. However, I shall be content here to speak of those that serve my purpose.

  The difference between hard bodies and those that are liquids is the first thing I would like you to note. To that end, consider that every body can be divided into extremely small parts. I do not wish to determine whether their number is infinite or not; at least it is certain that, with respect to our knowledge, it is indefinite and that we can suppose that there are several millions in the smallest grain of sand our eyes can perceive.

  Note also that, if two of these small parts are touching one another, without being in the act12 of moving away from one another, some force is necessary to separate them, however small it may be. For, once so placed, they would never be inclined to dispose themselves otherwise. Note also that twice as much force is necessary to separate two of them than to separate one of them, and a thousand times as much to separate a thousand of them. Thus, if it is necessary to separate several millions of them all at once, as is perhaps necessary in order to break a single hair, it is not surprising that a rather sensible force is necessary.

  By contrast, if two or more of these small parts touch one another only in passing and while they are in the act of moving, one in one direction and the other in another, certainly it will require less force to separate them than if they were in fact without motion. Indeed, no force at all will be required if the motion with which they are able to separate themselves is equal to or greater than that with which one wishes to separate them. Now, I find no difference between hard bodies and liquid bodies other than that the parts of the one can be separated from the whole much more easily than those of the other. Thus, to constitute the hardest body imaginable, I think it is enough if all the parts touch each other with no space remaining between any two and with none of them being in the act of moving. For what glue or cement can one imagine beyond that to hold them better one to the other?

  I think also that to constitute the most liquid body one could find, it is enough if all its smallest parts are moving away from one another in the most diverse ways and as quickly as possible, even though in that state they do not cease to be able to touch one another on all sides and to arrange the~selves in as small a space as if they were without motion. Finally, I believe that every body more or less approaches these extremes, according as its parts are more or less in the act of moving away from one another. All the phenomena on which I cast my eye confirm me in this opinion.

  Since, as I have already said, all the parts of flame are perpetually agitated, not only is it liquid, but it also renders most other bodies liquid. Note also that, when it melts metals, it acts with no different power than when it burns wood. Rather, because the parts of metals are just about all equal, the flame cannot move one part without moving the other, and hence it forms completely liquid bodies from them. By contrast, the parts of wood are unequal in such a way that the flame can separate the smaller of them and render them liquid (i.e. cause them to fly away in smoke) without agitating the larger parts.

  After flame, there is nothing more liquid than air, and one can see with the eye that its parts move separately from one another. For, if you take the effort to watch those small bodies that are commonly called “atoms” and that appear in rays of sunlight, you will see them flutter about incessantly here and there in a myriad of different ways, even when there is no wind stirring them up. One can also experience the same sort of thing in all the grosser liquids if one mixes them together in different colors, in order better to distinguish their motions. Finally, the phenomenon appears very clearly in acids13 when they move and separate the parts of some metal.

  But you could ask me here at this point why, if it is only the motion of the parts of flame that cause it to burn and make it liquid, the motion of the parts of air, which also make it extremely liquid, do not at all give it the power to burn but, on the contrary, make it such that our hands can hardly feel it? To this I reply that one must take into account not only the speed of motion, but also the size of the parts. It is the smaller ones that make the more liquid bodies, but it is the larger ones that have more force to burn and in general to act on other bodies.

  Note in passing that here, and always hereafter, I take a single part to be everything that is joined together and is not in the act of separation, even though the smallest parts could easily be divided into many other smaller ones. Thus, a grain of sand, a stone, a rock, indeed the whole earth itself, may hereafter be taken as a single part insofar as we are there considering only a completely simple and completely equal motion.14

  Now if, among the parts of air, there are some that are very large in comparison with the others (as are the “atoms” that are seen there), they also move very slowly; and, if there are some that move more quickly, they are also smaller. If, however, among the parts of flame there are some smaller than in air, there are also larger ones, or at least there is a larger number of parts equal to the largest of those of air. In addition, these larger parts of flame move much more quickly, and hence it is they alone that have the power to burn.

  As far as the smaller parts are concerned, one may guess th
at they penetrate many bodies of which the pores are so narrow that even air cannot enter. As far as the larger parts are concerned (or the equally large parts in greater number), one sees clearly how air alone does not suffice to nourish flame. The violence of their action is enough to show us that they move more quickly. Finally, that it is the largest of these parts that have the power to burn, and not the others, is apparent from the fact that the flame that issues from brandy, or from other very subtle bodies, hardly burns at all, while on the contrary that which is engendered in hard and heavy bodies is very hot.

  CHAPTER FOUR On the Void, and How it Happens that Our Senses Are Not Aware of Certain Bodies

  But we must examine in greater detail why air, although it is as much a body as the others, cannot be sensed as well as they. By doing so, we will free ourselves from an error with which we have been preoccupied since childhood, when we believed that there were no other bodies around us except those that could be sensed and thus that, if air were one of them, then, because we sensed it so faintly, it at least could not be as material nor as solid as those we sense more clearly.15

  On this subject, I would first like you to note that all bodies, both hard and liquid, are made from the same matter, and that it is impossible to conceive of the parts of that matter ever composing a more solid body, or one occupying less space, than they do when each of them is touched on all sides by the others surrounding it. Whence it seems to me to follow that, if there can be a void anywhere, it ought to be in hard bodies rather than liquid ones; for it is evident that the parts of the latter can much more easily press and arrange themselves against one another (because they are moving) than can those of the former (which are without motion).

  For example, if you are placing powder in a jar, you shake the jar and pound against it to make room for more powder. But, if you are pouring some liquid into it, the liquid spontaneously arranges itself in as small a place as one can put it. By the same token, if you consider in this regard some of the experiments the philosophers have been wont to use in showing that there is no void in nature,16 you will easily recognize that all those spaces that people think to be empty, and where we feel only air, are at least as full, and as full of the same matter, as those where we sense other bodies.

  For pray tell me what reason would there be to think that nature would cause the heaviest bodies to rise and the most solid to break — as one experiences her doing in certain machines, rather than to suffer that any of their parts should cease to touch one another or to touch some other bodies — and yet permit the parts of air — which are so easy to bend and to be arranged in all manners — to remain next to one another without being touched on all sides, or even without there being another body among them that they touch? Could one really believe that the water in a well should mount upward against its natural inclination merely in order that the pipe of a pump may be filled and [yet] think that the water in clouds should not fall in order that the spaces here below be filled, if there were even some little void among the parts of the bodies that they contain?17

  But you could propose to me here a rather considerable problem, to wit, that the parts composing liquid bodies cannot, it seems, move incessantly, as I have said they do, unless there is some empty space among them, at least in the places from which they depart by virtue of their being in motion. I would have trouble responding to this, had I not recognized through various experiences that all the motions that take place in the world are in some way circular. That is to say, when a body leaves its place, it always enters into that of another, and the latter into that of still another, and so on down to the last which occupies in the same instant the place left open by the first.18 Thus, there is no more of a void among them when they are moving than when they are stopped. And note here that it is not thereby necessary that all the parts of bodies that move together be exactly disposed in the round, as in a true circle, nor even that they be of equal size and shape; for these inequalities can easily be compensated for by other inequalities to be found in their speed.

  Now, when bodies move in the air, we do not usually notice these circular motions, because we are accustomed to conceiving of the air only as an empty space. But look at fish swimming in the pool of a fountain: if they do not approach too near to the surface of the water, they cause great speed. Whence it clearly appears that the water they push before them does not push indifferently all the water of the pool, but only that which can best serve to perfect the circle of the fishes’ motion and return to the place they leave behind.19 This experience suffices to show how these circular motions are easy for nature and familiar to her.

  Now, however, I want to adduce another experience to show that no motion ever takes place that is not circular. When the wine in a cask does not flow through an opening at the bottom because the top is completely closed, it is improper to say (as one ordinarily does) that this takes place owing to horror vacui. One well knows that the wine has no mind to fear anything; and, even if it had one, I do not know for what reason it might fear that void, which is in fact nothing but a chimera. Rather, one should say that the wine cannot leave the cask because outside everything is as full as it can be and that the part of the air, whose place the wine would occupy should it descend, cannot find another place to put itself anywhere in the rest of the universe unless one makes an opening in the top of the cask, through which this air can rise circularly to its place.

  Nevertheless, I do not want to make certain that there is no void at all in nature. I fear my discourse would become too long if I undertook to unfold the whole story, and the experiences of which I have spoken are not sufficient to prove it, although they are enough to persuade us that the spaces where we sense nothing are filled with the same matter, and contain at least as much of that matter, as those occupied by the bodies that we sense. Thus, for example, when a vessel is full of gold or lead, it nonetheless contains no more matter than when we think it is empty. This may well seem strange to many whose [powers of] reasoning do not extend beyond their fingertips and who think there is nothing in the world except what they touch. But when you have considered for a bit what makes us sense a body or not sense it, I am sure you will find nothing incredible in the above. For you will know clearly that, far from all the things around us being sensible, it is on the contrary those that are there most of the time that can be sensed the least, and those that are always there that can never be sensed at all.

  The heat of our heart is quite great, but we do not feel it because it is always there. The weight of our body is not small, but it does not discomfort us. We do not even feel the weight of our clothes because we are accustomed to wearing them. The reason for this is clear enough; for it is certain that we cannot sense any body unless it is the cause of some change in our sensory organs, i.e. unless it moves in some way the small parts of the matter of which those organs are composed. The objects that are not always present can well do this, provided only that they have force enough; for, if they corrupt something there while they act, that can be repaired afterward by nature, when they are no longer acting. But if those that continually touch us ever had the power to produce any change in our senses, and to move any parts of their matter, in order to move them they had perforce to separate them entirely from the others at the beginning of our life, and thus they can have left there only those that completely resist their action and by means of which they cannot be sensed in any way. Whence you see that it is no wonder that there are many spaces about us in which we sense no body, even though they contain bodies no less than those in which we sense them the most.

  But one need not therefore think that the coarse air that we draw into our lungs while breathing, that is converted into wind when agitated, that appears solid when enclosed in a balloon, and that is composed only of exhalations and smoke is as solid as water or earth. Here one should follow the common opinion of the philosophers, who all assure us that it is rarer, as one also easily recognizes from experience. For the parts of a drop of
water, separated from one another by the agitation of heat, can make up much more of this air than the space that held the water can contain. Whence it follows most certainly that there is a great quantity of small intervals among the parts of which the air is composed; for there is no other way to conceive of a rare body. But, because these intervals cannot be empty, as I have said above, I conclude from all this that of necessity there are mixed with the air some other bodies, either one or several, which fill as exactly as possible the small intervals left among its parts. Now there remains to consider only what these other bodies can be; thereafter I hope it will not be difficult to understand what the nature of light can be.

  CHAPTER FIVE On the Number of Elements and on Their Qualities

  The philosophers assure us that there is above the clouds a certain air much subtler than ours. That air is not composed of vapors of the earth as it is, but constitutes an element in itself. They say also that above this air there is still another, much subtler body, which they call the element of fire. They add, moreover, that these two elements are mixed with water and earth in the composition of all the inferior bodies. Thus~ I am only following their opinion if I say that this subtler air and this element of fire fill the intervals among the parts of the grosser air we breathe, so that these bodies, interlaced with one another, compose a mass as solid as any body can be.

  But, in order better to make you understand my thought on this subject, and so that you will not think I want to force you to believe all the philosophers tell us about the elements, I should describe them to you in my fashion.

  I conceive of the first, which one may call the element of fire, as the most subtle and penetrating fluid there is in the world. And in consequence of what has been said above concerning the nature of liquid bodies, I imagine its parts to be much smaller and to move much faster than any of those of other bodies. Or rather, in order not to be forced to admit any void in nature, I do not attribute to this first element parts having any determinate size or shape; but I am persuaded that the impetuosity of their motion is sufficient to cause it to be divided, in every way and in every sense, by collision with other bodies and that its parts change shape at every moment to accommodate themselves to the shape of the places they enter. Thus, there is never a passage so narrow, nor an angle so small, among the parts of other bodies, where the parts of this element do not penetrate without any difficulty and which they do not fill exactly.20