I would be quite happy to continue and to show here the whole chain of other truths that I have deduced from these first ones. But because, in order to do this, it would now be necessary for me to speak about many questions that are a matter of controversy among the learned, with whom I have no desire to get into any quarrel, I believe it will be better for me to abstain from this and to state only in a general way what these questions are, in order to let those who are wiser judge whether it would be useful for the public to be more particularly informed about them. I have always remained firm in the resolution I had made not to suppose any principle but the one I have just used to demonstrate the existence of God and of the soul, and not to accept anything as true that did not seem to me clearer and more certain than the demonstrations of the geometers had hitherto seemed. And, nevertheless, I dare say not only that have I found a means of satisfying myself within a short time regarding all the principal difficulties commonly treated in philosophy, but also that I have noted certain laws that God has so established in nature, and of which he has impressed in our souls such notions, that, after having reflected sufficiently on these matters, we cannot doubt that they are strictly adhered to in everything that exists or occurs in the world. Moreover, in considering the consequences of these laws, it seems to me that I have discovered many truths more useful and more important than all that I had previously learned or even hoped to learn.
But because I have tried to explain the principal ones among these truths in a treatise that certain considerations prevented me from publishing , I could not make them better known than by stating here in summary form what the treatise contains. I had intended to include in it everything that I thought I knew, before writing it down, concerning the nature of material things. But just as painters, who are unable to represent equally well on a flat surface all the various sides of a solid body, choose one of the principal sides which they place alone facing the light of day, and, by darkening the rest with shadows, make them appear only as they can be seen by someone who is looking at the principal side, just so, fearing I could not put into my discourse everything I had in mind about it, I undertook in it merely to speak at length about what I conceived with respect to light; then, at the proper time, to add something about the sun and the fixed stars, because light proceeds almost entirely from them; something about the heavens, because they transmit light; about planets, comets, and the earth, because they reflect light; and, in particular, about all terrestrial bodies, because they are either colored, or transparent, or luminous; and finally, about man, because he is the observer of these things. All the same, to cast all these things a little in shadow and to be able to say more freely what I judged about them without being obliged either to follow or to refute the opinions that are accepted among the learned, I resolved to leave this entire world here to their disputes, and to speak only of what would happen in a new world, were God now to create enough matter to compose it, somewhere in imaginary spaces, and were he to agitate in various ways and without order the different parts of this matter, so that he composed from it a chaos as confused as any the poets could concoct and that later he did no more than apply his ordinary concurrence to nature, and let nature act in accordance with the laws he had established. Thus, first, I described this matter and tried to represent it in such a way that there is nothing in the world, it seems to me, clearer and more intelligible, with the exception of what has already been said about God and the soul, for I even explicitly supposed that in this matter there were none of those forms or qualities about which disputes occur in the schools, nor generally anything the knowledge of which was not so natural to our souls that one could not even pretend to be ignorant of it. Moreover, I showed what the laws of nature were, and, without supporting my reasons on any other principle but the infinite perfections of God, I tried to demonstrate all those laws about which one might have been able to have any doubt and to show that they are such that, even if God had created many worlds, there could not be any of them in which these laws failed to be observed. After that, I showed how, as a consequence of these laws, the greater part of the matter of this chaos had to be disposed and arranged in a certain way, which made it similar to our heavens; how, at the same time, some of its parts had to compose an earth; others, planets and comets; and still others, a sun and fixed stars. And here, dwelling on the subject of light, I explained at some length what this light was that had to be found in the sun and the stars, and how from thence it traveled in an instant across the immense spaces of the heavens, and how it was reflected from the planets and comets to the earth. To this I added also a number of things touching on the substance, position, motions, and all the various qualities of these heavens and these stars; and as a result, I thought I said enough on these matters to show that there is nothing to be observed in the things of this world which should not, or at least could not, have appeared entirely similar in those of the world I was describing. From there, I went on to speak in particular about the earth: how, although I had expressly supposed that God had not put any weight  in the matter out of which the earth was composed, none of its parts ceased to tend precisely toward its center; how, there being water and air on its surface, the disposition of the heavens and of the stars, principally of the moon, had to cause there an ebb and flow similar in all respects to what we observe in our seas, and, in addition, a certain coursing, as much of the water as of the air, from east to west, such as is also observed between the tropics; how mountains, seas, springs, and rivers could naturally be formed there, and how metals could make their way into mines there; how plants could grow naturally in the fields there, and generally how all the bodies called "mixed" or "composed" could be engendered there. And, among other things, because apart from the stars I know of nothing else in the world that would produce light except fire, I tried to make very clearly understood all that belonged to its nature: how it is made, how it is nourished, how sometimes it has only heat but no light, and sometimes only light but no heat; how it can introduce various colors and various other qualities into various bodies; how it melts some bodies and hardens others; how it can consume nearly all of them or turn them into ashes and smoke; and finally, how from these ashes, merely by the force of its action, it produces glass, for since this transmutation of ashes into glass seemed to me to be as awesome as any other that occurs in nature, I took particular pleasure in describing it.
Yet I did not want to infer from all these things that this world has been created in the manner I was proposing, for it is much more likely that, from the beginning, God made it such as it had to be. But it is certain (and this is an opinion commonly accepted among theologians) that the action by which God preserves the world is precisely the same as that by which he created it; so that, even if, in the beginning, he had never given it any other form at all but that of a chaos, provided he established the laws of nature and bestowed his concurrence in order for nature to function just as it does ordinarily, one can believe, without doing injustice to the miracle of creation, that by this means alone all the things that are purely material could over time have been rendered such as we now see them. And their nature is much easier to conceive, when one sees them coming to be little by little in this manner, than when one considers them only in their completed state.
From the description of inanimate bodies and plants I passed to that of animals and in particular to that of human beings. But because I did not yet have sufficient knowledge of them to speak of them in the same manner as I did of the rest, that is to say, by demonstrating effects from causes and by showing from what seeds and in what manner nature must produce them, I contented myself with supposing that God formed the body of a man exactly like one of ours, as much in the outward shape of its members as in the internal arrangement of its organs, without composing it out of any material but the type I had described, and without putting into it, at the start, any rational soul, or anything else to serve there as a vegetative or sensitive soul, but merely kindled in the man's heart one of those fires without light which I had already explained and which I did not at all conceive to be of a nature other than what heats hay when it has been stored before it is dry, or which makes new wines boil when they are left to ferment after crushing. For on examining the functions that could, as a consequence, be in this body, I found there precisely all those things that can be in us without our thinking about them, and hence, without our soul's contributing to them, that is to say, that part distinct from the body of which it has been said previously that its nature is only to think. And these are all the same features in which one can say that animals lacking reason resemble us. But I could not on that account find there any of those functions, which, being dependent on thought, are the only ones that belong to us as men, although I did find them all later on, once I had supposed that God created a rational soul and joined it to this body in a particular manner that I described.
But in order that one might be able to see how I treated this matter there, I want to place here the explanation of the movement of the heart and of the arteries, because, this being the first and most general movement that one observes in animals, on the basis of it one will easily judge what one ought to think about all the others. And, in order that there might be less difficulty in understanding what I shall say on the matter, I would like those who are not at all versed in anatomy to take the trouble, before reading this, to have the heart of some large animal that has lungs dissected in their presence (for such a heart is in all respects sufficiently similar to that of a man), and to be shown the two chambers or cavities that are in it. First, there is the one on the right side of the heart, into which two very large tubes lead, namely the vena cava, which is the principal receptacle of the blood, and which is like the trunk of a tree of which the other veins of the body are the branches, and the arterial vein (which has thus been rather ill- named, because it is, in effect, an artery), which, taking its origin from the heart, divides up after leaving the heart into many branches that go on to be spread throughout the lungs. Then there is the chamber or cavity on the left side, into which two tubes lead in the same fashion, which are as large as or larger than the preceding ones: namely, the venous artery (which has also been ill-named, since it is nothing but a vein), which comes from the lungs, where it is divided into many branches interlaced with those of the arterial vein and with those in the passageway called the windpipe, through which the air one breathes enters, and the great artery, which, on leaving the heart, sends its branches throughout the body. I would also like those who are not versed in anatomy to be carefully shown the eleven little membranes that, like so many little doors, open and shut the four openings in the two cavities: namely, three at the entrance to the vena cava, where they are so disposed that they cannot in any way prevent the blood it contains from flowing into the right cavity of the heart, and yet completely prevent it from being able to leave it: three at the entrance to the arterial vein, which, being arranged totally in the other direction, readily permit the blood in this cavity to pass into the lungs, but do not permit any blood in the lungs to return there; likewise, two others at the entrance to the venous artery, which let blood flow from the lungs into the left cavity of the heart but block its return; and three at the entrance to the great artery, which permit blood to leave the heart but prevent it from returning there. And there is no need at all to search for any other reason for the number of membranes except that the opening of the venous artery, being. oval- shaped because of its location, can conveniently be closed with two, while the other openings, being round, can better be closed with three. Further, I would like to make them consider that the great artery and the arterial vein are of a much harder and firmer constitution than the venous artery and the vena cava, and that these latter two become enlarged before entering the heart and there form, as it were, sacks, called the "auricles" of the heart, which are made of flesh similar to that of the heart; and that there is always more heat in the heart than anywhere else in the body, and, finally, that this heat is able to bring it about that, if a drop of blood enters its cavities, it promptly expands and is dilated, just as all liquids generally do when one lets them fall drop by drop into some vessel that is very hot.
For, after that, I have no need to say anything. else in order to explain the movement of the heart, except that, when its cavities are not full of blood, blood necessarily flows from the vena cava into the right cavity and from the venous artery into the left cavity, given that these two vessels are always full of blood, and their openings, which face the heart, cannot then be closed. But as soon as two drops of blood have thus entered the heart, one into each of its cavities, these drops, which can only be very large because the openings through which they enter are very wide and the vessels from whence they come are quite full of blood, are rarified and dilated because of the heat they find there, by means of which, making the whole heart inflate, they push and close the five little doors that are at the entrances to the two vessels from whence they come, thus preventing any more blood from descending into the heart, and, continuing to become more and more rarified, they push and open the six other little doors which are at the entrances to the other two vessels by which they leave. By this means they inflate all the branches of the arterial vein and the great artery, almost at the same instant as the heart; immediately afterward the heart contracts, as do these arteries as well, because the blood that has entered them gets cooled and their six little doors close again, and the five doors of the vena cava and the venous artery reopen and grant passage to two other drops of blood, which immediately make the heart and the arteries inflate exactly as before. And, because the blood that thus enters the heart passes through the two sacks called its auricles, it follows from this that their movement is contrary to that of the heart, and that they are deflated while the heart is inflated. As for the rest (in order that those who do not know the force of mathematical demonstrations and are not accustomed to distinguishing true reasons from probable ones should not venture to deny this without examining it), I want to put them on notice that this movement which I have just been explaining follows just as necessarily from the mere disposition of the organs that can be seen in the heart by the naked eye, and from the heat that can be felt with the fingers, and from the nature of blood, which can be known through observation, as does the movement of a clock from the force, placement, and shape of its counterweights and wheels.
But if one asks how it is that the blood in the veins is not at all dissipated in flowing thus continually into the heart, and how the arteries are never overly full of blood, since all the blood that flows through the heart is going to flow into them, to this I need give no other answer than what has already been written by an English physician,  to whom homage must be paid for having broken the ice in this area, and for being the first to have taught that there are many small passages at the extremities of the arteries through which the blood they receive enters into the small branches of the veins, from which it flows immediately to the heart, so that its course is merely a perpetual circulation. He proves this very effectively from the common experience of surgeons, who, on binding an arm moderately tightly above the spot where they open the vein, cause the blood to flow out in even greater abundance than if they had not bound the arm at all. And just the opposite would happen if they bound the arm below, between the hand and the opening, or even if they bind it very tightly above the opening, for it is obvious that a moderately tight tourniquet, being able to prevent the blood that is already in the arm from returning to the heart through the veins, does not on that account prevent new blood from coming in through the arteries, because they are located below the veins, and their membranes, being harder, are less easy to press, and also because the blood coming from the heart tends to pass through the arteries toward the hand with greater force than it does in returning from these to the heart through the veins. And since this blood leaves the arm through the opening in one of the veins, there must necessarily be some passages below the tourniquet, that is to say, toward the extremities of the arm, through which it could come from the arteries. He also proves quite effectively what he says regarding the circulation of blood by referring to certain small membranes that are so disposed in various places along the length of the veins that they do not at all permit blood to pass from the middle of the body toward the extremities, but only to return from the extremities toward the heart; and further, by means of the experiment that shows that all the blood that is in the body can flow out of it in a very short time through just one artery when it is cut open, even if the artery is very tightly bound quite close to the heart, and cut open between the heart and the tourniquet, so that one would have no basis for imagining that the blood that flowed out came from somewhere else.
But there are many other things that attest to the fact that the true cause of this movement of blood is as I have said. First, the difference that one notices between the blood leaving the veins and the blood leaving the arteries can result only from the fact that the blood is rarified and, as it were, distilled, in passing through the heart; it is thinner, livelier , and warmer just after having left the heart, that is to say, while it is in the arteries, than it is shortly before it enters the heart, that is to say, while it is in the veins. And if one takes note of it, one will find that this difference is more readily apparent near the heart and not at all so much in those places furthest removed from the heart. Then the hardness of the membranes of which the arterial vein and the great artery are composed shows well enough that the blood beats against them with more force than it does against the veins. And why would the left cavity of the heart and the great artery be larger and wider than the right cavity and the arterial vein, unless it is because the blood in the venous artery, having been only in the lungs after having passed through the heart, is thinner and is more forcefully and more easily rarified than what comes immediately from the vena cava ? And what can physicians divine from taking the pulse, if they do not know that, as the blood changes its nature, it can be rarified by the heat of the heart more or less strongly, and more or less quickly than before? And if one examines how this heat is communicated to the other members, must one not admit that it is by means of the blood, which, on passing through the heart, is reheated there and from there is spread throughout the whole body? It follows from this that if one removes the blood from some part of the body, one thereupon also removes the heat; and even if the heart were as hot as a piece of glowing iron, it would not be enough to reheat the feet and hands as much as it does, if it did not continuously send new blood to them. Then, too, it is also evident from this that the true function of respiration is to bring enough fresh air into the lungs to cause the blood which comes there from the right cavity of the heart, where it has been rarified and, as it were, changed into vapors, immediately to be condensed and to be converted once again into blood before returning to the left cavity; without this process the blood could not properly aid in feeding the fire that is in the heart. This is confirmed because one sees that animals without lungs have but one single cavity in their hearts, and that children, who cannot use their lungs while enclosed within their mother's womb, have an opening through which blood flows from the vena cava into the left cavity of the heart, as well as a tube through which blood goes from the arterial vein to the great artery without passing through the lungs. Next, how would digestion take place in the stomach if the heart did not send heat there through the arteries, and with it some of the most fluid parts of the blood, which help dissolve the food that has gone there? And is it not easy to understand the action that changes the juice of this food into blood, if one considers that, in passing and repassing through the heart, it is distilled perhaps more than one or two hundred times a day? And is anything else needed to explain nutrition and the production of the various humors that are in the body, except to say that the force with which the blood, in being rarified, passes from the heart toward the extremities of the arteries, makes some of its parts stop in those parts of the members where they are found and there take the place of others that they expel from there; and that, according to the situation or the shape or the smallness of the pores they encounter, some of the parts of the blood tend to go certain places rather than others, in just the same way that anyone can have seen various sieves of different fineness serve to separate out different grains from one another? And finally what is most remarkable in all this is the generation of the animal spirits, which are like a very subtle wind, or rather, like a very pure and lively flame that rises continuously in great abundance from the heart to the brain, and from there goes through the nerves into the muscles, and gives movement to all the members. The parts of the blood that are the most agitated and penetrating, and are thus the best suited to compose these spirits, are going to move toward the brain rather than elsewhere; and there is no need to imagine any reason for this other than that the arteries that carry these parts of the blood there are those that come from the heart in the straightest line of all, and that, according to the laws of mechanics (which are the same as those of nature), when a number of things tend to move together in the same direction, where there is not enough room for all of them, as when the parts of the blood leaving the left cavity of the heart tend toward the brain, the weakest and least agitated must be pushed aside by the strongest which by this means arrive there alone.
I had provided a sufficiently detailed explanation for all these things in the treatise that I had previously intended to publish. And then I had shown what the constitution of the nerves and muscles of the human body must be in order to make the animal spirits within them have the force to move its members, as when one observes that heads, shortly after being severed, still move about and bite the earth, even though they are no longer alive. I had also shown what changes must take place in the brain in order to cause wakefulness, sleep, and dreams; how light, sounds, odors, tastes, heat, and all the other qualities of external objects can imprint various ideas there through the mediation of the senses; how hunger, thirst, and the other internal passions can also send their ideas there; what part of them needs to be taken there for the common sense, where these ideas are received, for the memory, which preserves them, and for the imagination, which can change them in various ways and compose new ones out of them, and, by the same means, distributing the animal spirits into the muscles, make the members of this body move in as many different ways (and in a manner appropriate to the objects that present themselves to the senses and to the internal passions that are in the body), as our own bodies can, without their being guided by the will. This will in no way seem strange to those who are cognizant of how many different automata or moving machines the ingenuity of men can make, without, in doing so, using more than a very small number of parts, in comparison with the great multitude of bones, muscles, nerves, arteries, veins, and all the other parts which are in the body of each animal. For they will regard this body as a machine which, having been made by the hands of God, is incomparably better ordered and has within itself movements far more wondrous than any of those that can be invented by men.
And I paused here in particular in order to show that, if there were such machines having the organs and the shape of a monkey or of some other animal that lacked reason, we would have no way of recognizing that they were not entirely of the same nature as these animals; whereas, if there were any such machines that bore a resemblance to our bodies and imitated our actions as far as this is practically feasible, we would always have two very certain means of recognizing that they were not at all, for that reason, true men. The first is that they could never use words or other signs, or put them together as we do in order to declare our thoughts to others. For one can well conceive of a machine being so made that it utters words, and even that it utters words appropriate to the bodily actions that will cause some change in its organs (such as, if one touches it in a certain place, it asks what one wants to say to it, or, if in another place, it cries out that one is hurting it, and the like). But it could not arrange its words differently so as to respond to the sense of all that will be said in its presence, as even the dullest men can do. The second means is that, although they might perform many tasks very well or perhaps better than any of us, such machines would inevitably fail in other tasks; by this means one would discover that they were acting not through knowledge but only through the disposition of their organs. For while reason is a universal instrument that can be of help in all sorts of circumstances, these organs require some particular disposition for each particular action; consequently, it is for all practical purposes impossible for there to be enough different organs in a machine to make it act in all the contingencies of life in the same way as our reason makes us act.
Now by these two means one can also know the difference between men and beasts. For it is rather remarkable that there are no men so dull and so stupid (excluding not even the insane), that they are incapable of arranging various words together and of composing from them a discourse by means of which they might make their thoughts understood, and that, on the other hand, there is no other animal at all, however perfect and pedigreed it may be, that does the like. This does not happen because they lack the organs, for one sees that magpies and parrots can utter words just as we can, and yet they cannot speak as we do, that is to say, by testifying to the fact that they are thinking about what they are saying; on the other hand, men born deaf and dumb, who are deprived of the organs that aid others in speaking just as much as, or more than beasts, are wont to invent for themselves various signs by means of which they make themselves understood to those who, being with them on a regular basis, have the time to learn their language. And this attests not merely to the fact that beasts have less reason than men but that they have none at all. For it is obvious it does not need much to know how to speak; and since we notice as much inequality among animals of the same species as among men, and that some are easier to train than others, it is unbelievable that a monkey or a parrot that is the most perfect of its species would not equal in this respect one of the most stupid children or at least a child with a disordered brain, if their soul were not of a nature entirely different from our own. And we should not confuse words with the natural movements that attest to the passions and can be imitated by machines as well as by animals. Nor should we think, as did some of the ancients, that beasts speak, although we do not understand their language, for if that were true, since they have many organs corresponding to our own, they could make themselves as well understood by us as they are by their fellow creatures. It is also a very remarkable phenomenon that, although there are many animals that show more skill than we do in some of their actions, we nevertheless see that they show none at all in many other actions. Consequently, the fact that they do something better than we do does not prove that they have any intelligence, for, were that the case, they would have more of it than any of us and would excel us in everything. But rather it proves that they have no intelligence at all, and that it is nature that acts in them, according to the disposition of their organs--just as we see that a clock composed exclusively of wheels and springs can count the hours and measure time more accurately than we can with all our carefulness.
After that, I described the rational soul and showed that it can in no way be derived from the potentiality of matter, as can the other things I have spoken of, but rather that it must be expressly created; and how it is not enough for it to be lodged in the human body like a pilot in his ship, unless perhaps in order to move its members, but rather that it must be more closely joined and united to the body in order to have, in addition to this, feelings and appetites similar to our own, and thus to constitute a true man. As to the rest, I elaborated here a little on the subject of the soul because it is of the greatest importance; for, after the error of those who deny the existence of God (which I think I have sufficiently refuted), there is none at all that puts weak minds at a greater distance from the straight path of virtue than to imagine that the soul of beasts is of the same nature as ours, and that, as a consequence, we have nothing to fear or to hope for after this life any more than do flies and ants. On the other hand, when one knows how different they are, one understands much better the arguments which prove that our soul is of a nature entirely independent of the body, and consequently that it is not subject to die with it. Then, since we do not see any other causes at all for its destruction, we are naturally led to judge from this that it is immortal.
7. Descartes' Le Monde (The World). See Rene Descartes, Le Monde ou Traite de la lumiere, translation and introduction by Michael Sean Mahoney (New York: Abaris Books, Inc., 1979). One of the considerations preventing the publication of Le Monde was the trial in 1633 of Galileo by the Holy Office in Rome.
8. E. Gilson, in his Discours de la methode: texte et commentaire, p. 388, observes that pesanteur here means the same thing as gravitas, a scholastic term referring to the tendency of terrestrial objects always to tend downwards. Gilson also directs the reader to The World, chapter xi: "On Weight."
9. William Harvey (1578-1657), English physiologist who demonstrated the function of the heart and the complete circulation of blood throughout the body. His most important work is Anatomical Exercises on the Motion of the Heart and Blood (1628). Descartes accepted Harvey's account of how blood circulated, but not his account of the heart's motion.