Third Cause of Changes in the Animal Spirits

The third cause of the changes that occur in the animal spirits is the most ordinary and the most active of all; because it is this cause that produces, sustains, and strengthens all the passions. To understand it well, one must know that the fifth, sixth, and eighth pairs of nerves send most of their branches into the chest and the abdomen, where they have uses very useful for the preservation of the body, but extremely dangerous for the soul; because these nerves do not depend in their action on the will of men, as do those that serve to move the arms, the legs, and the other external parts of the body, and because they act much more upon the soul than the soul acts upon them.

On the change of the spirits caused by the nerves that go to the heart and the lungs

One must therefore know that several branches of the eighth pair of nerves insert themselves between the fibers of the principal of all muscles, which is the heart; that they surround its openings, its auricles, and its arteries; that they spread even into the substance of the lung, and that thus, by their different movements, they produce very considerable changes in the blood. For the nerves that are spread between the fibers of the heart, sometimes causing it to expand and contract with too much force and quickness, drive with extraordinary violence a quantity of blood toward the head and toward all the external parts of the body; sometimes also these same nerves produce a wholly contrary effect. As for the nerves that surround the openings of the heart, its auricles, and its arteries, they produce roughly the same effect as the registers with which chemists moderate the heat of their furnaces, and the taps used in fountains to regulate the flow of their waters. For the use of these nerves is to constrict and dilate in various ways the openings of the heart, to hasten and retard in this manner the entry and exit of the blood, and thus to increase and diminish its heat. Finally, the nerves that are spread through the lung also have the same use; for the lung being composed only of the branches of the trachea, the pulmonary vein, and the pulmonary artery interlaced with one another, it is evident that the nerves spread through its substance, by their contraction, prevent the air from passing with sufficient freedom from the branches of the trachea, and the blood from those of the pulmonary vein, into the pulmonary artery to reach the heart. Thus these nerves, according to their different agitation, further increase or diminish the heat and movement of the blood.

We have in all our passions very perceptible experiences of these different degrees of heat in our heart. We manifestly feel it diminish and sometimes increase all at once; and because we falsely judge that our sensations are in the parts of our body on the occasion of which they are excited in our soul, as was explained in the first book, almost all philosophers have imagined that the heart was the principal seat of the passions of the soul, and this is even today the most common opinion.

Now, because the faculty of imagining receives great changes from those that occur in the animal spirits, and because the animal spirits are very different according to the different fermentation or agitation of the blood that takes place in the heart, it is easy to recognize what makes passionate persons imagine things quite differently from those who consider them in cold blood.

On that which is caused by the nerves that go to the liver, the spleen, and the viscera. — III. That all this happens against our will, but that it cannot happen without a providence.

II. The other cause, which greatly contributes to diminishing and increasing these extraordinary fermentations of the blood, consists in the action of several other branches of the nerves of which we have just spoken.

These branches spread into the liver, which contains the most subtle part of the blood, or what is ordinarily called bile; into the spleen, which contains the grossest part, or melancholy; into the pancreas, which contains an acid juice very suitable, it seems, for fermentation; into the stomach, the intestines, and the other parts that contain the chyle; finally, they spread into all the places that can contribute something to vary the fermentation or movement of the blood. There are even the arteries and veins that are bound by these nerves, as Mr. Willis has discovered concerning the lower trunk of the great artery, which is bound near the heart, the axillary artery on the right side, the emulgent vein, and some others.

Thus, the use of the nerves being to agitate in various ways the parts to which they are attached, it is easy to conceive how, for example, the nerve that surrounds the liver can, by constricting it, cause a great quantity of bile to flow into the veins and into the bile duct, which, having mixed with the blood in the veins, and with the chyle through the bile duct, enters the heart and produces a much more ardent heat there than usual. Thus, when one is moved by certain passions, the blood boils in the arteries and veins; the heat spreads throughout the body; the fire rises to the head, and it fills with so great a number of animal spirits too lively and too agitated that, by their impetuous course, they prevent the imagination from representing to itself anything other than those things of which they form images in the brain — that is to say, from thinking about any objects other than those of the passion that dominates.

It is the same with the small nerves that go to the spleen or to other parts that contain a grosser matter less susceptible to heat and movement; they make the imagination wholly languid and drowsy, by causing some gross matter, difficult to set in motion, to flow into the blood.

As for the nerves that surround the arteries and veins, their use is to prevent the blood from passing, and to oblige it, by constricting them, to flow into places where it finds free passage. Thus, the part of the great artery that supplies blood to all the parts below the heart, being bound and constricted by these nerves, the blood must necessarily enter the head in greater abundance, and thus produce change in the animal spirits, and consequently in the imagination.

III. It must be carefully noted that all this happens only by mechanism — that is to say, all these different movements of these nerves in all the different passions do not occur by the command of the will, but happen on the contrary without its orders, and even against its orders; so that a body without a soul, disposed like that of a healthy man, would be capable of all the movements that accompany our passions. Thus even beasts can have similar ones, even if they were only pure machines.

This is what should make us admire the incomprehensible wisdom of Him who has so well arranged all these springs, that it suffices for an object to move the optic nerve lightly in such and such a manner to produce so many diverse movements in the heart, in the other interior parts of the body, and even on the face. For it has recently been discovered that the same nerve that spreads some branches into the heart and into the other interior parts also communicates some of its branches to the eyes, the mouth, and the other parts of the face. So that no passion can arise within that does not appear outwardly, because there can be no movement in the branches that go to the heart without some movement occurring in those that are spread over the face.

The correspondence and sympathy that exists between the nerves of the face and some others that correspond to other parts of the body that cannot be named is even more remarkable; and what causes this great sympathy is, as in the other passions, that the small nerves that go to the face are still only branches of the one that descends lower down.

When one is surprised by some violent passion, if one takes care to reflect upon what one feels in the entrails and in the other parts of the body where the nerves insinuate themselves, as well as upon the changes of countenance that accompany it; and if one considers that all these diverse agitations of our nerves are entirely involuntary, and that they happen even despite all the resistance that our will brings against them, one will have little difficulty in allowing oneself to be persuaded by the simple exposition just given of all these relationships between the nerves.

But if one examines the reasons and the end of all these things, one will find so much order and wisdom there that a somewhat serious attention will be capable of convincing the persons most attached to Epicurus and Lucretius that there is a providence that governs the world. When I see a watch, I am right to conclude that there is an intelligence, because it is impossible that chance could have produced and arranged all its wheels. How, then, would it be possible that chance and the collision of atoms could have been capable of arranging in all men and all animals so many diverse springs, with the exactness and proportion that I have just explained, and that men and animals should engender others entirely similar to themselves? Thus it is ridiculous to think or to say, like Lucretius, that chance formed all the parts that compose man; that eyes were not made to see, but that one happened to see because one had eyes; and so on for the other parts of the body. Here are his words:

Lumina ne facias oculorum clara creata Prospicere ut possimus, et ut proferre viai Proceros passus, ideo fastigia posse Surarum ac feminum pedibus fundata plicari: Brachia tum porro validis exapta lacertis Esse, manusque datas utraque ex parte ministras Ut facere ad vitam possimus, quæ foret usus. Cætera de genere hoc inter quæcumque pretantur Omnia perversa præpostera sunt ratione. Nil ideo natum est in nostro corpore ut uti Possimus; sed quod natum est, id procreat usum.

Must one not have a strange aversion to providence in order to blind oneself so voluntarily for fear of recognizing it, and to try to render oneself insensible to proofs as strong and as convincing as those that nature furnishes us with? It is true that when one once affects to play the freethinker, or rather the impious, as the Epicureans did, one finds oneself immediately covered in darkness, and one sees only false glimmers; one boldly denies the clearest things, and one proudly and magisterially asserts the falsest and most obscure.

The poet I have just cited can serve as proof of this blindness of freethinkers. For he pronounces boldly and against all appearance of truth on the most difficult and obscure questions, and he seems not to perceive even the clearest and most evident ideas. If I were to pause to quote passages from this author to justify what I say, I would make too long and too tedious a digression. If it is permissible to make some reflections that arrest the mind for a moment upon essential truths, it is never permissible to make digressions that divert the mind for a considerable time from attention to its principal subject, in order to apply it to things of little importance.

We have just explained the general causes, both external and internal, that produce change in the animal spirits, and consequently in the faculty of imagining. We have shown that the external ones are the foods by which we are nourished and the air that we breathe, and that the internal one consists in the involuntary agitation of certain nerves. No other general causes are known, and I even assure that there are none. So that the faculty of imagining depending, on the body’s side, only on these two things — namely, the animal spirits and the disposition of the brain upon which they act — there remains here only, in order to give a perfect knowledge of the imagination, to set forth the different changes that can occur in the substance of the brain. But before examining these changes, it is appropriate to explain the connection of our thoughts with the traces of the brain, and the reciprocal connection of these traces. It will also be necessary to give some idea of memory and of habits — that is to say, of that facility we have of thinking about things we have already thought about, and of doing things we have already done.