The nature of sensations

The third thing found in each of our sensations, or what we feel—for example, when we are near a fire—is a modification of our soul in relation to what is happening in the body to which it is united.

This modification is pleasant when what is happening in the body helps the circulation of the blood and the other functions of life: we call it by the equivocal term “heat”; and this modification is painful and entirely different from the other when what is happening in the body is capable of inconveniencing or burning it—that is, when the motions within the body are capable of breaking some of its fibers—and it is ordinarily called pain or burning, and so on for other sensations. But here are the common thoughts people have on this subject.

We know sensations better than we think

II. The first error is believing that we have no knowledge of our sensations. Every day, countless people trouble themselves greatly to know what pain, pleasure, and other sensations are; they do not agree that these exist only in the soul and are merely modifications of it. It is true that such people are remarkable for wanting to be taught what they cannot possibly ignore, for it is impossible for a man to be entirely ignorant of what pain is when he feels it. A person, for example, who burns their hand distinguishes very well the pain they feel from light, color, sound, tastes, smells, pleasure, and from any other pain than the one they are feeling; they distinguish it very well from admiration, desire, love; they distinguish it from a square, a circle, motion; finally, they recognize it as very different from all things that are not the pain they are feeling. Now, if they had no knowledge of pain, I would like to know how they could know with evidence and certainty that what they feel is none of these things.

We therefore know, in some way, what we immediately feel when we see colors or have some other sensation, and it is even very certain that if we did not know it, we would know no sensible object at all; for it is evident that we could not distinguish, for example, water from wine if we did not know that the sensations we have of one are different from those we have of the other, and so for all things we know through the senses.

Objection and response

It is true that if pressed, and asked to explain what pain, pleasure, color, etc., are, I could not do so properly in words; but it does not follow from this that if I see color or burn myself, I do not know, at least in some way, what I am currently feeling.

Now, the reason why all sensations cannot be well explained by words, like all other things, is that it depends on human will to attach the ideas of things to whatever names they please. They can call the sky Ouranos, Schamajim, etc., as the Greeks and Hebrews did; but these same men do not attach their sensations to words, nor to anything else, as they please. They see no color though one speaks of it to them, if they do not open their eyes; they taste no flavors unless some change occurs in the arrangement of the fibers of their tongue and brain. In short, all sensations do not depend on human will, and only He who made them preserves them in this mutual correspondence between the modifications of their soul and those of their body. So that if a man wants me to represent heat or color to him, I cannot use words for this purpose; but I must imprint upon the organs of his senses the motions to which nature has attached these sensations; I must bring him near the fire and show him paintings.

It is for this reason that it is impossible to give the blind the slightest knowledge of what is meant by red, green, yellow, etc. For, since one cannot make oneself understood when the listener does not have the same ideas as the speaker, it is manifest that colors being attached not to the sound of words or to the motion of the ear nerve, but to that of the optic nerve, one cannot represent them to the blind, since their optic nerve cannot be stirred by colored objects.

Why we imagine we know nothing of our sensations

We therefore have some knowledge of our sensations. Let us now see why we still seek to know them and believe we have no knowledge of them. Here is undoubtedly the reason.

Since the Fall, the soul has become, as it were, corporeal by inclination. Its love for sensible things continually diminishes the union or relationship it has with intelligible things. It conceives things that are not felt only with disgust, and it quickly tires of considering them. It makes every effort to produce in its brain some images that represent them; and it has become so accustomed from childhood to this kind of conception that it believes it does not even know what it cannot imagine. However, there are several things which, not being corporeal, cannot be represented to the mind by corporeal images, such as our soul with all its modifications. When, therefore, our soul wishes to represent its nature and its own sensations to itself, it strives to form a corporeal image of them; it searches for itself among all corporeal beings; it takes itself sometimes for one, sometimes for another; sometimes for air, sometimes for fire, or for the harmony of the parts of its body, and, wishing thus to find itself among bodies and to imagine its own modifications, which are its sensations, as modifications of bodies, it is no wonder if it goes astray and completely misrecognizes itself.

What further leads it to want to imagine its sensations is that it judges them to be in objects and to be even modifications of them, and, consequently, to be something corporeal and imaginable. It therefore judges that the nature of its sensations consists only in the motion that causes them, or in some other modification of a body; which is found to be different from what it feels, which is nothing corporeal and cannot be represented by corporeal images. This embarrasses it and makes it believe that it does not know its own sensations.

As for those who make no vain efforts to represent the soul and its modifications by corporeal images, yet still ask to have sensations explained to them, they should know that the soul and its modifications are not known through ideas, taking the word “idea” in its true sense, as I determine and explain it in the third book, but only through inner sentiment; and, thus, when they wish to have the soul and its sensations explained to them through some ideas, they wish for what it is not possible for all men together to give them, since men cannot instruct us by giving us the ideas of things, but only by making us think of those we naturally possess.

The second error into which we fall regarding sensations is that we attribute them to objects. This was explained in chapters XI and XII.

We wrongly believe that all men have the same sensations of the same objects

The third error is that we judge that everyone has the same sensations of the same objects. We believe, for example, that everyone sees the sky as blue, the meadows as green, and all visible objects in the same way that we see them, and so for all other sensible qualities of the other senses. Many people will even be surprised that I doubt things they consider indubitable. However, I can assure them that they have never had any reason to judge in the manner they do, and although I cannot mathematically demonstrate that they are mistaken, I can nevertheless demonstrate that if they are not mistaken, it is by the greatest chance in the world. I even have strong enough reasons to assure that they are truly in error. To recognize the truth of what I advance, one must remember what I have already proved, namely, that there is a great difference between sensations and the causes of sensations. For one can judge from this that, strictly speaking, it is possible that similar motions of the inner fibers of the optic nerve do not produce in different persons the same sensations—that is, seeing the same colors—and that it may happen that a motion which causes in one person the sensation of blue will cause that of green or gray in another, or even a new sensation that no one has ever had.

It is certain that this can be so and that there is no reason demonstrating the contrary. However, it is agreed that it is not probable that it is so. It is much more reasonable to believe that God always acts in the same way in the union He has established between our souls and our bodies, and that He has attached the same ideas and the same sensations to similar motions of the inner fibers of the brain in different persons.

Let it then be true that the same motions of the fibers terminating in the brain are accompanied by the same sensations in all men; if it happens that the same objects do not produce the same motions in their brains, they will consequently not excite the same sensations in their souls. Now it seems to me indubitable that since the sense organs of all men are not disposed in the same manner, they cannot receive the same impressions from the same objects.

The punches that porters give each other in play would be capable of crippling delicate persons. The same blow produces very different motions, and consequently excites very different sensations in a man of robust constitution and in a child or a woman of weak complexion. Thus, since there are not two people in the world whose sense organs can be assured to be in perfect conformity, one cannot assure that there are two men in the world who have exactly the same feelings about the same objects.

This is the origin of that strange variety encountered in human inclinations. Some love music extremely, others are insensitive to it; and even among those who enjoy it, some love one genre, others another, according to the almost infinite diversity found in the fibers of the hearing nerve, in the blood, and in the spirits.

How much difference, for example, is there between French music, Italian music, Chinese music, and others, and consequently between the taste different peoples have for different genres of music! It even happens that at different times one receives very different impressions from the same concerts; for if one’s imagination is heated by a great abundance of agitated spirits, one takes much more pleasure in hearing bold music with many dissonances than in softer music that follows rules and mathematical exactness. Experience proves this, and it is not very difficult to give the reason.

It is the same with smells. He who loves the orange blossom may perhaps not tolerate the rose, and others the contrary. As for tastes, there is as much diversity as in other sensations. Sauces must be all different to please equally different people, or to please the same person equally at different times. One likes sweet, another likes sour. One finds wine pleasant and another has a horror of it; and the same person who finds it pleasant when in good health finds it bitter when having a fever, and so on for the other senses. However, all men love pleasure; they all love pleasant sensations; they all have the same inclination in this respect. They therefore do not receive the same sensations from the same objects, since they do not love them equally.

Thus, what makes a man say that he likes sweetness is that the sensation he has of it is pleasant; and what makes another say that he does not like sweetness is that, in truth, he does not have the same sensation as the one who likes it. And then when he says he does not like sweetness, it does not mean that he does not like having the same sensation as the other, but only that he does not have it. So that one speaks improperly when saying one does not like sweetness; one should say that one does not like sugar, honey, etc., which all others find sweet and pleasant, and which one does not find to taste the same as others because one has the fibers of the tongue differently disposed.

Here is a more sensitive example: suppose that among twenty people there is one who has cold hands, and who does not know the names used in France to explain the sensations of cold and heat, and that all the others, on the contrary, have extremely hot hands. If in winter one brought them all slightly lukewarm water to wash, those who had very hot hands, washing first one after another, might well say: “This water is very cold, I do not like this.” But when the last one, who has extremely cold hands, comes at the end to wash, he would say on the contrary: “I do not know why you do not like cold water; for my part, I take pleasure in feeling the cold and in washing.”

It is quite clear in this example that when this last one says: “I like the cold,” it signifies nothing other than that he likes warmth and feels it where the others feel the contrary. Thus when a man says: “I like what is bitter, and I cannot stand sweets,” it signifies nothing other than that he does not have the same sensations as those who say they like sweets and have an aversion to everything that is bitter.

It is therefore certain that a sensation which is pleasant to one person is also so to all who feel it; but that the same objects do not make everyone feel it, due to the different disposition of the sense organs: which is of the utmost consequence to remark for physics and for morals.

Objection and response

One can only raise here an objection very easy to resolve, namely: that it sometimes happens that people who extremely loved certain meats finally come to have a horror of them, either because while eating them they found some dirt mixed in which surprised them, or because they became very ill from having eaten them in excess, or finally for other reasons. Such people, it will be said, no longer love the same sensations they once loved, for they still have them when they eat the same meats, and yet they are no longer pleasant to them.

To respond to this objection, one must note that when these people taste meats for which they have such horror and disgust, they have two very different sensations at the same time. They have that of the meat they are eating, as the objection supposes, and they also have another sensation of disgust, which comes, for example, from their strongly imagining the dirt they saw mixed with what they are eating. The reason for this is that when two motions have occurred in the brain at the same time, one is no longer excited without the other, except after a considerable time. Thus, because the pleasant sensation never comes without this other disgusting one, and because we confuse things that happen at the same time, we imagine that this sensation which was formerly pleasant is no longer so. However, if it is always the same, it is necessary that it always be pleasant. So that if one imagines it is not pleasant, it is because it is joined and confused with another which causes more disgust than the former has agreeableness.

There is more difficulty in proving that colors and some other sensations, which I have called weak and languid, are not the same in all men, because all these sensations touch the soul so little that one cannot distinguish, as in tastes or other stronger and more vivid sensations, that one is more pleasant than the other, and thus recognize, by the variety of pleasure or disgust found in different persons, the diversity of their sensations. Nevertheless, the reason which shows that other sensations are not similar in different persons also shows that there must be variety in the sensations one has of colors. Indeed, one cannot doubt that there is much diversity in the visual organs of different persons, as well as in those of hearing or taste; for there is no reason to suppose a perfect resemblance in the disposition of the optic nerve of all men, since there is an infinite variety in all things of nature, and principally in those that are material. There is therefore some appearance that all men do not see the same colors in the same objects. However, I believe it never happens, or almost never, that persons see white and black as a different color than we do, although they do not see them as equally white or black. But for intermediate colors, such as red, yellow, and blue, and principally those composed of these three, I believe there are very few people who have exactly the same sensation of them. For there are sometimes people who see certain bodies as yellow, for example, when they look at them with one eye, and as green or blue when they look at them with the other. However, if one supposed that these people were born one-eyed, or with eyes disposed to see blue what is called green, they would believe they see objects of the same color as we see them, because they would always have heard named green what they saw as blue.

One could further prove that all men do not see the same objects as the same color, because, according to the remarks of some, the same colors do not please all sorts of people equally; since if these sensations were the same, they would be equally pleasant to all men. But because very strong objections can be made against this proof, supported by the response I gave to the previous objection, it is not considered solid enough to propose.

Indeed, it is quite rare that one takes much more pleasure in one color than in another, just as one takes much more pleasure in one taste than in another. The reason is that the sentiments of colors are not given to us to judge whether bodies are suitable for our nourishment or not. This is marked by pleasure and pain, which are the natural characters of good and evil. Objects as colored are neither good nor bad to eat. If objects appeared to us as pleasant or unpleasant as colored, their sight would always be followed by the flow of spirits which excites and accompanies the passions, since one cannot touch the soul without moving it. We would often hate good things and love bad ones, so that we would not preserve our lives for long. Finally, sentiments of color are given to us only to distinguish bodies from one another, and this is done just as well whether one sees grass as green or as red, provided that the person who sees it green or red always sees it in the same manner.

But enough has been said of these sensations; let us now speak of natural judgments and free judgments which accompany them. This is the fourth thing that we confuse with the three others we have just treated.