I would here willingly have proceeded to exhibit the whole chain of truths which I deduced from these primary but as with a view to this it would have been necessary now to treat of many questions in dispute among the earned, with whom I do not wish to be embroiled, I believe that it will be better for me to refrain from this exposition, and only mention in general what these truths are, that the more judicious may be able to determine whether a more special account of them would conduce to the public advantage. I have ever remained firm in my original resolution to suppose no other principle than that of which I have recently availed myself in demonstrating the existence of God and of the soul, and to accept as true nothing that did not appear to me more clear and certain than the demonstrations of the geometers had formerly appeared; and yet I venture to state that not only have I found means to satisfy myself in a short time on all the principal difficulties which are usually treated of in philosophy, but I have also observed certain laws established in nature by God in such a manner, and of which he has impressed on our minds such notions, that after we have reflected sufficiently upon these, we cannot doubt that they are accurately observed in all that exists or takes place in the world and farther, by considering the concatenation of these laws, it appears to me that I have discovered many truths more useful and more important than all I had before learned, or even had expected to learn.
I would gladly present here the whole chain of truths I deduced from these first principles. But doing so would require discussing many questions disputed among scholars, and I do not wish to become embroiled with them. So I believe it is better to leave out that explanation and describe these truths only in general terms. More discerning readers may then judge whether a detailed account would benefit the public. I have always held firmly to my original decision to assume no other principle than the one I recently used. With that principle, I demonstrated the existence of God and of the soul. I also resolved to accept nothing as true unless it seemed clearer and more certain than geometrical proofs once seemed to me. Yet I venture to say that I soon found ways to satisfy myself about all the main difficulties usually discussed in philosophy. I have also recognized certain laws that God established in nature, and of which he has given our minds certain ideas. These laws and ideas are such that, after sufficient reflection, we cannot doubt that the laws are followed exactly. They hold for everything that exists or happens in the world. Furthermore, by considering how these laws are linked, I seem to have discovered many truths of greater usefulness and importance. They surpass everything I had learned before, or had even expected to learn.
But because I have essayed to expound the chief of these discoveries in a treatise which certain considerations prevent me from publishing, I cannot make the results known more conveniently than by here giving a summary of the contents of this treatise. It was my design to comprise in it all that, before I set myself to write it, I thought I knew of the nature of material objects. But like the painters who, finding themselves unable to represent equally well on a plain surface all the different faces of a solid body, select one of the chief, on which alone they make the light fall, and throwing the rest into the shade, allow them to appear only in so far as they can be seen while looking at the principal one; so, fearing lest I should not be able to compense in my discourse all that was in my mind, I resolved to expound singly, though at considerable length, my opinions regarding light; then to take the opportunity of adding something on the sun and the fixed stars, since light almost wholly proceeds from them; on the heavens since they transmit it; on the planets, comets, and earth, since they reflect it; and particularly on all the bodies that are upon the earth, since they are either colored, or transparent, or luminous; and finally on man, since he is the spectator of these objects. Further, to enable me to cast this variety of subjects somewhat into the shade, and to express my judgment regarding them with greater freedom, without being necessitated to adopt or refute the opinions of the learned, I resolved to leave all the people here to their disputes, and to speak only of what would happen in a new world, if God were now to create somewhere in the imaginary spaces matter sufficient to compose one, and were to agitate variously and confusedly the different parts of this matter, so that there resulted a chaos as disordered as the poets ever feigned, and after that did nothing more than lend his ordinary concurrence to nature, and allow her to act in accordance with the laws which he had established. On this supposition, I, in the first place, described this matter, and essayed to represent it in such a manner that to my mind there can be nothing clearer and more intelligible, except what has been recently said regarding God and the soul; for I even expressly supposed that it possessed none of those forms or qualities which are so debated in the schools, nor in general anything the knowledge of which is not so natural to our minds that no one can so much as imagine himself ignorant of it. Besides, I have pointed out what are the laws of nature; and, with no other principle upon which to found my reasonings except the infinite perfection of God, I endeavored to demonstrate all those about which there could be any room for doubt, and to prove that they are such, that even if God had created more worlds, there could have been none in which these laws were not observed. Thereafter, I showed how the greatest part of the matter of this chaos must, in accordance with these laws, dispose and arrange itself in such a way as to present the appearance of heavens; how in the meantime some of its parts must compose an earth and some planets and comets, and others a sun and fixed stars. And, making a digression at this stage on the subject of light, I expounded at considerable length what the nature of that light must be which is found in the sun and the stars, and how thence in an instant of time it traverses the immense spaces of the heavens, and how from the planets and comets it is reflected towards the earth. To this I likewise added much respecting the substance, the situation, the motions, and all the different qualities of these heavens and stars; so that I thought I had said enough respecting them to show that there is nothing observable in the heavens or stars of our system that must not, or at least may not appear precisely alike in those of the system which I described. I came next to speak of the earth in particular, and to show how, even though I had expressly supposed that God had given no weight to the matter of which it is composed, this should not prevent all its parts from tending exactly to its center; how with water and air on its surface, the disposition of the heavens and heavenly bodies, more especially of the moon, must cause a flow and ebb, like in all its circumstances to that observed in our seas, as also a certain current both of water and air from east to west, such as is likewise observed between the tropics; how the mountains, seas, fountains, and rivers might naturally be formed in it, and the metals produced in the mines, and the plants grow in the fields and in general, how all the bodies which are commonly denominated mixed or composite might be generated and, among other things in the discoveries alluded to inasmuch as besides the stars, I knew nothing except fire which produces light, I spared no pains to set forth all that pertains to its nature,--the manner of its production and support, and to explain how heat is sometimes found without light, and light without heat; to show how it can induce various colors upon different bodies and other diverse qualities; how it reduces some to a liquid state and hardens others; how it can consume almost all bodies, or convert them into ashes and smoke; and finally, how from these ashes, by the mere intensity of its action, it forms glass: for as this transmutation of ashes into glass appeared to me as wonderful as any other in nature, I took a special pleasure in describing it. I was not, however, disposed, from these circumstances, to conclude that this world had been created in the manner I described; for it is much more likely that God made it at the first such as it was to be. But this is certain, and an opinion commonly received among theologians, that the action by which he now sustains it is the same with that by which he originally created it; so that even although he had from the beginning given it no other form than that of chaos, provided only he had established certain laws of nature, and had lent it his concurrence to enable it to act as it is wont to do, it may be believed, without discredit to the miracle of creation, that, in this way alone, things purely material might, in course of time, have become such as we observe them at present; and their nature is much more easily conceived when they are beheld coming in this manner gradually into existence, than when they are only considered as produced at once in a finished and perfect state.
But I have tried to explain the chief discoveries in a treatise that certain considerations prevent me from publishing. So the most convenient way to make the results known is to summarize that treatise here. I intended to include everything I thought I knew about the nature of material objects before I began writing. But painters cannot represent every side of a solid object equally well on a flat surface. They choose one main side and let the light fall on that alone. They leave the rest in shadow, showing only what can be seen while looking at the main side. Similarly, I feared I could not include everything in my mind within my account. So I decided to explain only my views on light, though at considerable length. I would then take the opportunity to add something about the sun and fixed stars, since light comes almost entirely from them. I would discuss the heavens, since they transmit light, and the planets, comets, and earth, since they reflect light. In particular, I would discuss all bodies on earth, since they are either colored, transparent, or luminous. Finally, I would discuss man, since he observes these objects. I also wanted to leave this range of subjects somewhat in shadow and express my judgment about them more freely. I wanted to avoid having to accept or refute the opinions of scholars. So I decided to leave everyone here to their disputes and speak only about what would happen in a new world. Suppose God now created enough matter to form a world somewhere in imaginary space. Suppose he set the different parts of this matter moving in various, confused ways. The result would be a chaos as disordered as any the poets ever imagined. Suppose God then did nothing more than give nature his usual support, allowing nature to follow the laws he had established. On this assumption, I first described this matter. I tried to present it as clearly and intelligibly as anything could be, in my judgment. The only exception was what I had recently said about God and the soul. I expressly assumed that this matter had none of the forms or qualities so debated in the schools. More generally, I assumed it contained nothing whose nature was not so naturally understood that no one could even imagine being ignorant of it. I also set out the laws of nature. I based my reasoning on no principle other than God's infinite perfection. From that foundation, I tried to demonstrate all the laws about which there could be any doubt. I also tried to prove that, even if God had created more worlds, these laws must hold in every one. Next, I showed how most of the matter in this chaos must arrange itself according to these laws. The resulting arrangement would have the appearance of heavens. Meanwhile, some parts must form an earth, planets, and comets, while others must form a sun and fixed stars. At this point, I turned aside to discuss light. I explained at considerable length what the nature of the light in the sun and stars must be. I also explained how that light instantly crosses the immense spaces of the heavens. Then I explained how the planets and comets reflect the light toward the earth. I added much about the substance, position, movements, and all the different qualities of these heavens and stars. I thought I had said enough to account for everything observable in the heavens or stars of our system. Everything we observe must, or at least may, appear exactly the same in the system I described. Next, I discussed the earth in particular. I had expressly assumed that God gave no weight to the matter composing it. Yet I showed how this would not prevent all its parts from tending directly toward its center. I then considered the earth with water and air on its surface. I showed how the arrangement of the heavens and heavenly bodies, especially the moon, must cause tides. These tides would resemble those observed in our seas in every respect. The same arrangement must also cause a current of both water and air from east to west. Such a current is also observed between the tropics. I showed how mountains, seas, springs, and rivers might form naturally on this earth. I also showed how metals might be produced in mines and plants might grow in fields. More generally, I explained how all bodies commonly called mixed or composite might be generated. Among the discoveries I described, I took particular care to explain everything about the nature of fire. For apart from the stars, I knew of nothing but fire that produces light. I explained how fire is produced and sustained, and how heat sometimes occurs without light, and light without heat. I showed how fire can give different bodies various colors and other qualities. I explained how fire makes some bodies liquid and hardens others. I also explained how fire can consume almost all bodies, or turn them into ashes and smoke. Finally, I showed how fire, through the sheer intensity of its action, turns these ashes into glass. This transformation of ashes into glass seemed to me as wonderful as any other transformation in nature. So I took special pleasure in describing it. However, I was not inclined to conclude from these explanations that this world had been created as I described. It is much more likely that God made the world from the beginning as it was meant to be. But one thing is certain, and theologians commonly accept it. The action by which God now sustains the world is the same action by which he originally created it. So suppose God had initially given the world no form other than chaos. Suppose only that he had established certain laws of nature and supported nature in acting as it ordinarily does. We may then believe that purely material things could, through this process alone, have gradually become what we observe today. This belief takes nothing away from the miracle of creation. We understand the nature of material things much more easily when we see them gradually coming into existence this way. Their nature is harder to understand when we consider them only as things produced all at once, complete and perfect.
From the description of inanimate bodies and plants, I passed to animals, and particularly to man. But since I had not as yet sufficient knowledge to enable me to treat of these in the same manner as of the rest, that is to say, by deducing effects from their causes, and by showing from what elements and in what manner nature must produce them, I remained satisfied with the supposition that God formed the body of man wholly like to one of ours, as well in the external shape of the members as in the internal conformation of the organs, of the same matter with that I had described, and at first placed in it no rational soul, nor any other principle, in room of the vegetative or sensitive soul, beyond kindling in the heart one of those fires without light, such as I had already described, and which I thought was not different from the heat in hay that has been heaped together before it is dry, or that which causes fermentation in new wines before they are run clear of the fruit. For, when I examined the kind of functions which might, as consequences of this supposition, exist in this body, I found precisely all those which may exist in us independently of all power of thinking, and consequently without being in any measure owing to the soul; in other words, to that part of us which is distinct from the body, and of which it has been said above that the nature distinctively consists in thinking, functions in which the animals void of reason may be said wholly to resemble us; but among which I could not discover any of those that, as dependent on thought alone, belong to us as men, while, on the other hand, I did afterwards discover these as soon as I supposed God to have created a rational soul, and to have annexed it to this body in a particular manner which I described.
After describing inanimate bodies and plants, I turned to animals, especially human beings. But I did not yet know enough to explain them as I had explained the rest. That would mean tracing effects back to their causes and showing what materials and processes nature must use to produce them. So I was content to suppose that God formed a human body entirely like ours. Its limbs had the same outward shape, and its organs had the same internal structure. The body was made of the same matter I had described. I supposed that God initially placed no rational soul in the body. Nor did he give it any other principle to take the place of a vegetative or sensitive soul. He merely kindled in its heart one of those fires without light that I had already described. I thought this fire was no different from the heat in hay piled up before it is dry. Nor did I think it differed from the heat that causes new wine to ferment before being drained from the fruit. I then examined what functions might exist in this body as a result of these assumptions. I found precisely all the functions that can occur in us independently of any ability to think. These functions therefore owe nothing at all to the soul. By the soul, I mean the part of us distinct from the body, whose essential nature, as stated above, is thinking. Animals without reason may be said to resemble us entirely in these functions. But I could find none of the functions that depend on thought alone and belong to us as human beings. Later, however, I did find these functions as soon as I supposed that God had created a rational soul. I supposed that he had joined this soul to the body in a particular way, which I described.
But, in order to show how I there handled this matter, I mean here to give the explication of the motion of the heart and arteries, which, as the first and most general motion observed in animals, will afford the means of readily determining what should be thought of all the rest. And that there may be less difficulty in understanding what I am about to say on this subject, I advise those who are not versed in anatomy, before they commence the perusal of these observations, to take the trouble of getting dissected in their presence the heart of some large animal possessed of lungs (for this is throughout sufficiently like the human), and to have shown to them its two ventricles or cavities: in the first place, that in the right side, with which correspond two very ample tubes, viz., the hollow vein (vena cava), which is the principal receptacle of the blood, and the trunk of the tree, as it were, of which all the other veins in the body are branches; and the arterial vein (vena arteriosa), inappropriately so denominated, since it is in truth only an artery, which, taking its rise in the heart, is divided, after passing out from it, into many branches which presently disperse themselves all over the lungs; in the second place, the cavity in the left side, with which correspond in the same manner two canals in size equal to or larger than the preceding, viz., the venous artery (arteria venosa), likewise inappropriately thus designated, because it is simply a vein which comes from the lungs, where it is divided into many branches, interlaced with those of the arterial vein, and those of the tube called the windpipe, through which the air we breathe enters; and the great artery which, issuing from the heart, sends its branches all over the body. I should wish also that such persons were carefully shown the eleven pellicles which, like so many small valves, open and shut the four orifices that are in these two cavities, viz., three at the entrance of the hollow veins where they are disposed in such a manner as by no means to prevent the blood which it contains from flowing into the right ventricle of the heart, and yet exactly to prevent its flowing out; three at the entrance to the arterial vein, which, arranged in a manner exactly the opposite of the former, readily permit the blood contained in this cavity to pass into the lungs, but hinder that contained in the lungs from returning to this cavity; and, in like manner, two others at the mouth of the venous artery, which allow the blood from the lungs to flow into the left cavity of the heart, but preclude its return; and three at the mouth of the great artery, which suffer the blood to flow from the heart, but prevent its reflux. Nor do we need to seek any other reason for the number of these pellicles beyond this that the orifice of the venous artery being of an oval shape from the nature of its situation, can be adequately closed with two, whereas the others being round are more conveniently closed with three. Besides, I wish such persons to observe that the grand artery and the arterial vein are of much harder and firmer texture than the venous artery and the hollow vein; and that the two last expand before entering the heart, and there form, as it were, two pouches denominated the auricles of the heart, which are composed of a substance similar to that of the heart itself; and that there is always more warmth in the heart than in any other part of the body--and finally, that this heat is capable of causing any drop of blood that passes into the cavities rapidly to expand and dilate, just as all liquors do when allowed to fall drop by drop into a highly heated vessel.
But to show how I handled this subject there, I will explain the movement of the heart and arteries here. This is the first and most general movement observed in animals. It will therefore help us readily judge what to think of all their other movements. To make my explanation easier to understand, I offer some advice to readers unfamiliar with anatomy. Before reading these observations, they should arrange to watch the dissection of a large animal’s heart. The animal should have lungs, since its heart is sufficiently like the human heart in all respects. They should have its two ventricles, or cavities, shown to them. First, they should examine the cavity on the right side, which connects with two very large tubes. One is the hollow vein (vena cava), the main reservoir of blood. It is like the trunk of a tree whose branches are all the other veins in the body. The other is the arterial vein (vena arteriosa), an inappropriate name, since this vessel is actually just an artery. It begins in the heart and divides into many branches after leaving it. These branches soon spread throughout the lungs. Second, they should examine the cavity on the left side, which likewise connects with two tubes. These are as large as, or larger than, the two already described. One is the venous artery (arteria venosa), another inappropriate name, because this vessel is simply a vein from the lungs. In the lungs, it divides into many branches. These intertwine with the branches of the arterial vein and those of the windpipe, through which the air we breathe enters. The other tube is the great artery, which leaves the heart and sends branches throughout the body. I would also like these readers to be carefully shown the eleven thin membranes that act as small valves. These open and close the four openings in the heart’s two cavities. Three are at the entrance of the hollow vein. Their arrangement does not prevent blood in the vein from entering the right ventricle, but completely prevents it from flowing out. Three more are at the entrance to the arterial vein, arranged in exactly the opposite way. They readily let blood in the right cavity pass into the lungs, but prevent blood in the lungs from returning. Likewise, two others at the opening of the venous artery let blood from the lungs enter the heart’s left cavity. But they prevent that blood from returning to the lungs. Three at the opening of the great artery let blood leave the heart, but prevent it from flowing back. We need look no further for the reason behind the number of these membranes. Because of its position, the opening of the venous artery is oval and can be adequately closed with two membranes. The other openings are round, so three membranes close them more conveniently. I also want these readers to observe the texture of the great artery and the arterial vein. Both are much harder and firmer than the venous artery and the hollow vein. These last two vessels widen before entering the heart, forming what look like two pouches called the heart’s auricles. The auricles consist of material similar to the heart itself. Readers should also observe that the heart is always warmer than any other part of the body. Finally, this heat can make any drop of blood entering the cavities rapidly expand and swell. All liquids do the same when dropped, a little at a time, into a very hot vessel.
For, after these things, it is not necessary for me to say anything more with a view to explain the motion of the heart, except that when its cavities are not full of blood, into these the blood of necessity flows,--from the hollow vein into the right, and from the venous artery into the left; because these two vessels are always full of blood, and their orifices, which are turned towards the heart, cannot then be closed. But as soon as two drops of blood have thus passed, one into each of the cavities, these drops which cannot but be very large, because the orifices through which they pass are wide, and the vessels from which they come full of blood, are immediately rarefied, and dilated by the heat they meet with. In this way they cause the whole heart to expand, and at the same time press home and shut the five small valves that are at the entrances of the two vessels from which they flow, and thus prevent any more blood from coming down into the heart, and becoming more and more rarefied, they push open the six small valves that are in the orifices of the other two vessels, through which they pass out, causing in this way all the branches of the arterial vein and of the grand artery to expand almost simultaneously with the heart which immediately thereafter begins to contract, as do also the arteries, because the blood that has entered them has cooled, and the six small valves close, and the five of the hollow vein and of the venous artery open anew and allow a passage to other two drops of blood, which cause the heart and the arteries again to expand as before. And, because the blood which thus enters into the heart passes through these two pouches called auricles, it thence happens that their motion is the contrary of that of the heart, and that when it expands they contract. But lest those who are ignorant of the force of mathematical demonstrations and who are not accustomed to distinguish true reasons from mere verisimilitudes, should venture, without examination, to deny what has been said, I wish it to be considered that the motion which I have now explained follows as necessarily from the very arrangement of the parts, which may be observed in the heart by the eye alone, and from the heat which may be felt with the fingers, and from the nature of the blood as learned from experience, as does the motion of a clock from the power, the situation, and shape of its counterweights and wheels.
After these explanations, I need only add the following to explain the heart’s motion. When the heart’s chambers are not full of blood, blood necessarily flows into them. Blood flows from the hollow vein into the right chamber, and from the venous artery into the left. This happens because both vessels are always full of blood, and their openings toward the heart cannot then close. But as soon as two drops of blood enter, one into each chamber, the heat inside immediately thins and expands them. These drops must be very large because the openings are wide and the vessels supplying them are full of blood. As the drops expand, they cause the whole heart to expand. At the same time, the drops press shut the five small valves at the entrances of the two supplying vessels. Closing these valves prevents any more blood from entering the heart. As the blood continues to thin and expand, it pushes open six small valves in the openings of the other two vessels. The blood flows out through these vessels. This causes all the branches of the arterial vein and the great artery to expand almost simultaneously with the heart. Immediately afterward, the heart begins to contract, as do the arteries, because the blood that entered them has cooled. The six small valves close. The five valves of the hollow vein and the venous artery reopen, allowing two more drops of blood to enter. These drops cause the heart and arteries to expand again, as before. The blood entering the heart passes through two pouches called auricles. Because of this, the auricles move in the opposite way to the heart: when the heart expands, the auricles contract. Some people do not understand the force of mathematical demonstrations and are not used to distinguishing sound reasons from merely plausible ones. To prevent such people from daring to deny this explanation without examining it, I ask them to consider the following. The motion I have explained follows necessarily from the arrangement of the heart’s parts, visible to the naked eye. It also follows from the heat that can be felt with the fingers and the nature of blood known through experience. The heart’s motion follows from these things just as necessarily as a clock’s motion follows from its counterweights and wheels. The clock’s motion depends on their force, position, and shape.
But if it be asked how it happens that the blood in the veins, flowing in this way continually into the heart, is not exhausted, and why the arteries do not become too full, since all the blood which passes through the heart flows into them, I need only mention in reply what has been written by a physician of England, who has the honor of having broken the ice on this subject, and of having been the first to teach that there are many small passages at the extremities of the arteries, through which the blood received by them from the heart passes into the small branches of the veins, whence it again returns to the heart; so that its course amounts precisely to a perpetual circulation. Of this we have abundant proof in the ordinary experience of surgeons, who, by binding the arm with a tie of moderate straitness above the part where they open the vein, cause the blood to flow more copiously than it would have done without any ligature; whereas quite the contrary would happen were they to bind it below; that is, between the hand and the opening, or were to make the ligature above the opening very tight. For it is manifest that the tie, moderately straightened, while adequate to hinder the blood already in the arm from returning towards the heart by the veins, cannot on that account prevent new blood from coming forward through the arteries, because these are situated below the veins, and their coverings, from their greater consistency, are more difficult to compress; and also that the blood which comes from the heart tends to pass through them to the hand with greater force than it does to return from the hand to the heart through the veins. And since the latter current escapes from the arm by the opening made in one of the veins, there must of necessity be certain passages below the ligature, that is, towards the extremities of the arm through which it can come thither from the arteries. This physician likewise abundantly establishes what he has advanced respecting the motion of the blood, from the existence of certain pellicles, so disposed in various places along the course of the veins, in the manner of small valves, as not to permit the blood to pass from the middle of the body towards the extremities, but only to return from the extremities to the heart; and farther, from experience which shows that all the blood which is in the body may flow out of it in a very short time through a single artery that has been cut, even although this had been closely tied in the immediate neighborhood of the heart and cut between the heart and the ligature, so as to prevent the supposition that the blood flowing out of it could come from any other quarter than the heart.
But someone may ask why the blood in the veins does not run out, since it continually flows into the heart. And why do the arteries not become too full, since all the blood passing through the heart enters them? In reply, I need only mention what an English physician has written. He deserves the honor of having pioneered this subject. He was the first to teach that many small passages exist at the ends of the arteries. Through these passages, the blood received from the heart enters the small branches of the veins. From there, the blood returns to the heart. Its course thus amounts precisely to a continuous circulation. Surgeons’ everyday experience gives us abundant proof of this circulation. They tie a moderately tight band around the arm, above the place where they open a vein. This makes the blood flow more freely than it would without any band. The opposite would happen if they tied the band below the opening, between the hand and the opening. The opposite would also happen if they tied the band very tightly above the opening. The reason is clear. A moderately tightened band prevents blood already in the arm from returning to the heart through the veins. But the band cannot therefore prevent fresh blood from entering through the arteries. The arteries lie deeper than the veins, and their firmer walls are harder to compress. Also, blood from the heart flows through the arteries toward the hand with greater force. Its return from the hand to the heart through the veins is less forceful. And since this returning blood escapes through the opening in a vein, certain passages must exist below the band. These passages, toward the ends of the arm, must allow blood from the arteries to reach the veins. This physician also provides abundant support for his account of blood’s movement by pointing to thin membranes in the veins. These membranes are arranged like small valves at various points along the veins. They prevent blood from flowing from the center of the body toward the extremities. They allow blood only to return from the extremities to the heart. He also draws on experience showing that all the body’s blood can escape very quickly through a single cut artery. This can happen even if the artery has been tightly tied very close to the heart. The cut is made between the heart and the band. This rules out the possibility that the escaping blood could come from anywhere other than the heart.
But there are many other circumstances which evince that what I have alleged is the true cause of the motion of the blood: thus, in the first place, the difference that is observed between the blood which flows from the veins, and that from the arteries, can only arise from this, that being rarefied, and, as it were, distilled by passing through the heart, it is thinner, and more vivid, and warmer immediately after leaving the heart, in other words, when in the arteries, than it was a short time before passing into either, in other words, when it was in the veins; and if attention be given, it will be found that this difference is very marked only in the neighborhood of the heart; and is not so evident in parts more remote from it. In the next place, the consistency of the coats of which the arterial vein and the great artery are composed, sufficiently shows that the blood is impelled against them with more force than against the veins. And why should the left cavity of the heart and the great artery be wider and larger than the right cavity and the arterial vein, were it not that the blood of the venous artery, having only been in the lungs after it has passed through the heart, is thinner, and rarefies more readily, and in a higher degree, than the blood which proceeds immediately from the hollow vein? And what can physicians conjecture from feeling the pulse unless they know that according as the blood changes its nature it can be rarefied by the warmth of the heart, in a higher or lower degree, and more or less quickly than before? And if it be inquired how this heat is communicated to the other members, must it not be admitted that this is effected by means of the blood, which, passing through the heart, is there heated anew, and thence diffused over all the body? Whence it happens, that if the blood be withdrawn from any part, the heat is likewise withdrawn by the same means; and although the heart were as-hot as glowing iron, it would not be capable of warming the feet and hands as at present, unless it continually sent thither new blood. We likewise perceive from this, that the true use of respiration is to bring sufficient fresh air into the lungs, to cause the blood which flows into them from the right ventricle of the heart, where it has been rarefied and, as it were, changed into vapors, to become thick, and to convert it anew into blood, before it flows into the left cavity, without which process it would be unfit for the nourishment of the fire that is there. This receives confirmation from the circumstance, that it is observed of animals destitute of lungs that they have also but one cavity in the heart, and that in children who cannot use them while in the womb, there is a hole through which the blood flows from the hollow vein into the left cavity of the heart, and a tube through which it passes from the arterial vein into the grand artery without passing through the lung. In the next place, how could digestion be carried on in the stomach unless the heart communicated heat to it through the arteries, and along with this certain of the more fluid parts of the blood, which assist in the dissolution of the food that has been taken in? Is not also the operation which converts the juice of food into blood easily comprehended, when it is considered that it is distilled by passing and repassing through the heart perhaps more than one or two hundred times in a day? And what more need be adduced to explain nutrition, and the production of the different humors of the body, beyond saying, that the force with which the blood, in being rarefied, passes from the heart towards the extremities of the arteries, causes certain of its parts to remain in the members at which they arrive, and there occupy the place of some others expelled by them; and that according to the situation, shape, or smallness of the pores with which they meet, some rather than others flow into certain parts, in the same way that some sieves are observed to act, which, by being variously perforated, serve to separate different species of grain? And, in the last place, what above all is here worthy of observation, is the generation of the animal spirits, which are like a very subtle wind, or rather a very pure and vivid flame which, continually ascending in great abundance from the heart to the brain, thence penetrates through the nerves into the muscles, and gives motion to all the members; so that to account for other parts of the blood which, as most agitated and penetrating, are the fittest to compose these spirits, proceeding towards the brain, it is not necessary to suppose any other cause, than simply, that the arteries which carry them thither proceed from the heart in the most direct lines, and that, according to the rules of mechanics which are the same with those of nature, when many objects tend at once to the same point where there is not sufficient room for all (as is the case with the parts of the blood which flow forth from the left cavity of the heart and tend towards the brain), the weaker and less agitated parts must necessarily be driven aside from that point by the stronger which alone in this way reach it I had expounded all these matters with sufficient minuteness in the treatise which I formerly thought of publishing. And after these, I had shown what must be the fabric of the nerves and muscles of the human body to give the animal spirits contained in it the power to move the members, as when we see heads shortly after they have been struck off still move and bite the earth, although no longer animated; what changes must take place in the brain to produce waking, sleep, and dreams; how light, sounds, odors, tastes, heat, and all the other qualities of external objects impress it with different ideas by means of the senses; how hunger, thirst, and the other internal affections can likewise impress upon it divers ideas; what must be understood by the common sense (sensus communis) in which these ideas are received, by the memory which retains them, by the fantasy which can change them in various ways, and out of them compose new ideas, and which, by the same means, distributing the animal spirits through the muscles, can cause the members of such a body to move in as many different ways, and in a manner as suited, whether to the objects that are presented to its senses or to its internal affections, as can take place in our own case apart from the guidance of the will. Nor will this appear at all strange to those who are acquainted with the variety of movements performed by the different automata, or moving machines fabricated by human industry, and that with help of but few pieces compared with the great multitude of bones, muscles, nerves, arteries, veins, and other parts that are found in the body of each animal. Such persons will look upon this body as a machine made by the hands of God, which is incomparably better arranged, and adequate to movements more admirable than is any machine of human invention. And here I specially stayed to show that, were there such machines exactly resembling organs and outward form an ape or any other irrational animal, we could have no means of knowing that they were in any respect of a different nature from these animals; but if there were machines bearing the image of our bodies, and capable of imitating our actions as far as it is morally possible, there would still remain two most certain tests whereby to know that they were not therefore really men. Of these the first is that they could never use words or other signs arranged in such a manner as is competent to us in order to declare our thoughts to others: for we may easily conceive a machine to be so constructed that it emits vocables, and even that it emits some correspondent to the action upon it of external objects which cause a change in its organs; for example, if touched in a particular place it may demand what we wish to say to it; if in another it may cry out that it is hurt, and such like; but not that it should arrange them variously so as appositely to reply to what is said in its presence, as men of the lowest grade of intellect can do. The second test is, that although such machines might execute many things with equal or perhaps greater perfection than any of us, they would, without doubt, fail in certain others from which it could be discovered that they did not act from knowledge, but solely from the disposition of their organs: for while reason is an universal instrument that is alike available on every occasion, these organs, on the contrary, need a particular arrangement for each particular action; whence it must be morally impossible that there should exist in any machine a diversity of organs sufficient to enable it to act in all the occurrences of life, in the way in which our reason enables us to act. Again, by means of these two tests we may likewise know the difference between men and brutes. For it is highly deserving of remark, that there are no men so dull and stupid, not even idiots, as to be incapable of joining together different words, and thereby constructing a declaration by which to make their thoughts understood; and that on the other hand, there is no other animal, however perfect or happily circumstanced, which can do the like. Nor does this inability arise from want of organs: for we observe that magpies and parrots can utter words like ourselves, and are yet unable to speak as we do, that is, so as to show that they understand what they say; in place of which men born deaf and dumb, and thus not less, but rather more than the brutes, destitute of the organs which others use in speaking, are in the habit of spontaneously inventing certain signs by which they discover their thoughts to those who, being usually in their company, have leisure to learn their language. And this proves not only that the brutes have less reason than man, but that they have none at all: for we see that very little is required to enable a person to speak; and since a certain inequality of capacity is observable among animals of the same species, as well as among men, and since some are more capable of being instructed than others, it is incredible that the most perfect ape or parrot of its species, should not in this be equal to the most stupid infant of its kind or at least to one that was crack-brained, unless the soul of brutes were of a nature wholly different from ours. And we ought not to confound speech with the natural movements which indicate the passions, and can be imitated by machines as well as manifested by animals; nor must it be thought with certain of the ancients, that the brutes speak, although we do not understand their language. For if such were the case, since they are endowed with many organs analogous to ours, they could as easily communicate their thoughts to us as to their fellows. It is also very worthy of remark, that, though there are many animals which manifest more industry than we in certain of their actions, the same animals are yet observed to show none at all in many others: so that the circumstance that they do better than we does not prove that they are endowed with mind, for it would thence follow that they possessed greater reason than any of us, and could surpass us in all things; on the contrary, it rather proves that they are destitute of reason, and that it is nature which acts in them according to the disposition of their organs: thus it is seen, that a clock composed only of wheels and weights can number the hours and measure time more exactly than we with all our skin.
But many other facts show that I have identified the true cause of the blood’s movement. First, consider the difference between blood flowing from veins and blood flowing from arteries. This difference can arise only because blood expands and is, so to speak, distilled as it passes through the heart. Immediately after leaving the heart, blood is thinner, brighter, and warmer than shortly before entering it. In other words, blood in the arteries differs in these ways from blood in the veins. Close attention shows that this difference is very marked only near the heart. It is less obvious farther away. Next, the firmness of the walls of the arterial vein and the great artery provides sufficient evidence. Blood strikes these walls with greater force than it strikes the veins. And why are the heart’s left chamber and the great artery wider and larger than their counterparts? Why are they larger than the right chamber and the arterial vein? The reason must be that blood from the venous artery expands more easily and to a greater degree. This blood has passed through the heart and then been only in the lungs. It is therefore thinner than blood coming directly from the hollow vein. And what can physicians infer from feeling the pulse unless they understand how blood expands? As blood changes in nature, the heart’s warmth can make it expand more or less, and faster or slower. And if we ask how this heat reaches the other parts of the body, must we not admit the answer? Blood passes through the heart, is heated again there, and then spreads throughout the body. So when blood is removed from any part, heat is removed along with it. Even if the heart were as hot as glowing iron, it could not warm the hands and feet as it does now. To do so, the heart must continually send fresh blood to them. We can also see from this that breathing has a particular purpose. It brings enough fresh air into the lungs to thicken the blood flowing there from the heart’s right chamber. In that chamber, the blood has expanded and, so to speak, turned into vapor. The lungs turn this vapor back into blood before it enters the left chamber. Without this process, the blood would be unfit to feed the fire there. Two observations support this explanation. Animals without lungs also have only one chamber in the heart. And children in the womb cannot use their lungs. They have an opening through which blood flows from the hollow vein into the heart’s left chamber. They also have a tube carrying blood from the arterial vein into the great artery without passing through the lungs. Next, how could the stomach digest food unless the heart supplied heat through the arteries? Along with this heat, the arteries carry some of the blood’s more fluid parts, which help dissolve food. Is it not also easy to understand how the liquid extracted from food becomes blood? Consider that this liquid is distilled by passing through the heart repeatedly, perhaps more than one or two hundred times daily. And what more must we add to explain nourishment and the production of the body’s different humors? As blood expands, it moves forcefully from the heart toward the ends of the arteries. This force causes some parts of the blood to remain in the body parts they reach. There they replace other parts that they drive out. Depending on the position, shape, or small size of the pores they encounter, some blood components enter particular parts rather than others. Certain sieves work in the same way. Their different openings allow them to separate different kinds of grain. Finally, what deserves the most attention here is the production of the animal spirits. These spirits resemble a very fine wind, or rather a very pure and bright flame. They continually rise in great quantities from the heart to the brain. From there, they pass through the nerves into the muscles and move every part of the body. The blood components best suited to forming these spirits are the most active and penetrating. To explain why these components move toward the brain, we need suppose only one cause. The arteries carrying them there follow the straightest paths from the heart. The laws of mechanics are the same as the laws of nature. Under these laws, many objects may move at once toward a point without enough room for them all. This happens to blood components leaving the heart’s left chamber and moving toward the brain. The stronger components must push the weaker, less active components aside. In this way, only the stronger components reach the brain. I had explained all these matters in sufficient detail in the treatise I once considered publishing. Afterward, I had shown how the nerves and muscles of the human body must be constructed. Their structure must enable the animal spirits within the body to move its parts. We see something similar when freshly severed heads still move and bite the ground, although no longer alive. I had also explained what changes in the brain must produce waking, sleep, and dreams. I had shown how external objects impress different ideas on the brain through the senses. These include light, sounds, smells, tastes, heat, and all the other qualities of external objects. I had likewise shown how hunger, thirst, and other internal feelings can impress various ideas on the brain. I had explained what we must understand by the common sense (sensus communis), where these ideas are received. I had also explained the memory that retains them and the imagination that can change them in various ways. The imagination can combine these ideas into new ones. By the same means, it distributes animal spirits through the muscles and makes the body’s parts move. Such a body can move in as many different ways as our own body can without the will’s guidance. Its movements can also respond just as suitably to objects presented to its senses or to its internal feelings. None of this will seem strange to people familiar with the many movements performed by human-made automata, or moving machines. These machines use very few parts compared with the many bones, muscles, nerves, arteries, veins, and other animal body parts. Such people will regard the animal body as a machine made by the hands of God. Its arrangement is incomparably better than that of any human invention. It can also perform far more remarkable movements. Here I took particular care to explain a distinction. Suppose machines had exactly the same organs and outward appearance as an ape or another animal without reason. We would have no way to know that their nature differed in any respect from that of those animals. But suppose machines resembled our bodies and imitated our actions as closely as was practically possible. Two entirely reliable tests would still show that these machines were not truly human. The first test concerns words or other signs arranged to communicate our thoughts to others. Machines could never use these as we do. We can easily imagine a machine built to utter words. We can even imagine it uttering words in response to external objects that change its organs. For example, touching one spot might make the machine ask what we want to say to it. Touching another might make it cry out that it is hurt, and so on. But the machine could not arrange words in different ways to answer appropriately whatever is said in its presence. Even people with the least intelligence can do that. The second test concerns the range of a machine’s abilities. Such machines might perform many tasks as well as any of us, or perhaps even better. Yet they would undoubtedly fail at certain others. These failures would reveal that the machines acted only through the arrangement of their organs, not through understanding. Reason is a universal tool, equally available on every occasion. By contrast, these organs need a particular arrangement for each particular action. It must therefore be practically impossible for a machine to contain enough different organs to handle every situation in life. Our reason enables us to act in all such situations. These same two tests also let us distinguish human beings from animals. One fact deserves particular attention. No human beings are so dull or unintelligent that they cannot combine different words to make their thoughts understood. This holds even for people with severe intellectual disabilities. Yet no other animal can do the same, however perfect it is or however favorable its circumstances. This inability does not come from lacking the necessary organs. Magpies and parrots can utter words as we do. Yet they cannot speak as we do, in a way that shows they understand what they say. By contrast, people born deaf and unable to speak spontaneously invent signs to express their thoughts. They lack the use of the organs others employ in speaking at least as much as animals do, indeed more. Yet their signs communicate their thoughts to companions who have time to learn their language. This proves not merely that animals have less reason than human beings, but that they have none at all. We see that very little reason is needed for speech. Animals of the same species differ in ability, just as human beings do. Some animals are also more capable of being taught than others. So consider the most perfect ape or parrot of its species. It is incredible that this animal could not equal the least intelligent human child in speech. Or it should at least equal a child whose mind is disordered. This would be incredible unless animal souls had a nature entirely different from ours. We must not confuse speech with the natural movements that express emotions. Machines can imitate these movements, just as animals can display them. Nor should we believe, as some ancient thinkers did, that animals speak a language we do not understand. If that were true, animals could communicate their thoughts to us as easily as to one another. After all, they have many organs similar to ours. Another fact also deserves close attention. Many animals show greater skill than we do in certain actions. Yet those same animals show no skill at all in many others. Their superior performance in some things does not prove that they have minds. If it did, we would have to conclude that they possessed greater reason than any of us. They would then be able to surpass us in everything. Instead, their performance shows that they lack reason. Nature acts in them according to the arrangement of their organs. In the same way, a clock made only of wheels and weights can count hours and measure time. It does so more accurately than we can with all our skill.
I had after this described the reasonable soul, and shown that it could by no means be educed from the power of matter, as the other things of which I had spoken, but that it must be expressly created; and that it is not sufficient that it be lodged in the human body exactly like a pilot in a ship, unless perhaps to move its members, but that it is necessary for it to be joined and united more closely to the body, in order to have sensations and appetites similar to ours, and thus constitute a real man. I here entered, in conclusion, upon the subject of the soul at considerable length, because it is of the greatest moment: for after the error of those who deny the existence of God, an error which I think I have already sufficiently refuted, there is none that is more powerful in leading feeble minds astray from the straight path of virtue than the supposition that the soul of the brutes is of the same nature with our own; and consequently that after this life we have nothing to hope for or fear, more than flies and ants; in place of which, when we know how far they differ we much better comprehend the reasons which establish that the soul is of a nature wholly independent of the body, and that consequently it is not liable to die with the latter and, finally, because no other causes are observed capable of destroying it, we are naturally led thence to judge that it is immortal.
After this, I had described the rational soul. I had shown that it could not possibly arise from matter, as the other things I had discussed could. Instead, the soul must be specially created. Nor is it enough for the soul to occupy the human body like a pilot in a ship. Such an arrangement might perhaps allow the soul to move the body's limbs. But to have sensations and desires like ours, the soul must be more closely joined and united with the body. Only then can soul and body form a real human being. In conclusion, I discussed the soul at considerable length because the subject is of the greatest importance. I think I have already sufficiently refuted the error of those who deny God's existence. After that error, none leads weak minds away from the straight path of virtue more powerfully than the following belief: that animals' souls have the same nature as ours. From this belief follows the conclusion that, after this life, we have no more to hope for or fear than flies and ants. By contrast, understanding how greatly our souls differ from animals' souls helps us understand the reasons for the soul's independence. These reasons establish that the soul's nature is wholly independent of the body. Consequently, the soul is not subject to dying with the body. Finally, since we observe no other causes capable of destroying the soul, we are naturally led to judge it immortal.