Senses. The senses are a faculty of the soul through which it perceives external objects. This occurs by means of some action or impression made on certain parts of the body, called the sense organs, which communicate this impression to the brain.

Some people give a broader meaning to senses; they define them as a faculty through which the soul receives ideas or images of objects, whether this occurs by means of an impression made by these objects externally or by some action on the soul itself.

If one considers the senses from this point of view, two types have to be distinguished, external and internal senses; this corresponds to the two different ways in which the images of objects we perceive are produced and communicated to the mind. These images can come directly from the outside, that is, by means of the five external senses—hearing, sight, taste, touch, and smell—or they can be internal, produced by the inner senses, like imagination, memory, attention, to which can be added hunger, thirst, pain, etc.

The external senses are the means through which the soul can perceive or take cognizance of external objects. Such means can be considered from the point of view of the mind as well as from the point of view of the body. Mental capacities always remain the same. It is always the same faculty that makes it possible for us to see or to hear. The physical means are as varied as the objects we must perceive. This is why there are so many different organs of feeling. Each is constituted in such a way as to give the soul some representation and some information about the state of external objects, about their proximity, their fitness or unfitness, and other qualities. Moreover, the impressions vary according to the degree of distance, the proximity of danger, or the advantage to be derived. It is this which leads to the differing functions of the various organs, such as hearing, seeing, sniffing or smelling, tasting, touching.

An excellent modern author [3] gives a very ingenious explanation of the senses; according to his principles, the senses should be defined as the capacity to perceive or to receive ideas. Sometimes, instead of capacity, he prefers to call it the predisposition of the mind to receive ideas; he calls sensations the ideas which are perceived in this way or which arrive in the mind.

Thus, the external senses are the same thing as the capacity of receiving ideas under the stimulus of exterior objects. In such cases the soul is purely passive. It can neither prevent the perception nor the idea, nor vary the way in which it is received as long as the body remains capable of receiving impressions from external objects.

Whenever two perceptions are completely different from each other, or when they merely fall under the general related category of sensation, then we give the name of different senses to the capacity of the soul to receive perceptions which are so different from each other. Thus, sight and hearing denote the distinction between the capacity to receive ideas of color and of sound, in spite of the fact that there can be great variation in colors or sounds. Nevertheless even among the most disparate colors there exist greater interrelations than there can exist between a color and a sound; and that is why colors are considered perceptions of the same sense; all senses seem to have their particular organ except for the sense of touch. This sense is distributed over the entire body.

The internal senses are an energy or firmness of the mind. The mind depends upon ideas received through the perception of objects by the external senses. The internal senses are of two kinds, deriving either from pleasing and lovely forms of natural objects, or from beautiful actions.

If we reflect about the nature of our external senses, it becomes clear that the perception of pleasure or pain does not depend upon our will. Objects do not please us because we so wish it. There exist objects which are necessarily agreeable to us and others which we dislike in spite of ourselves. The only way in which we can, by our own volition, receive pleasure or avert evil is by acquiring pleasurable objects or by protecting ourselves from evil. It is in our nature to respond with pleasure to the one and uneasily to the other. As a matter of fact, the pleasant or unpleasant sensation is immediate without our knowing what caused it, without our awareness of the circumstances out of which it arose, and without the perception of the usefulness or inconvenience that may result from the use of these objects. Even the most perfect acquaintance with such things would not alter the sensation of pleasure or pain. Yet one might derive a rational pleasure, quite different from the pleasure of emotion, or feel satisfaction at the thought of advantages that can be expected from the object; one can also experience a sense of aversion at the mere apprehension of evil.

Almost all objects with which our soul concerns itself give rise to a feeling of well-being or anxiety; thus we may find ourselves pleasantly affected by a regular form, an architectural construction, a painting, a piece of music, and we have an inward feeling that this pleasure arises spontaneously from the contemplation of the idea that is then present in our mind together with all its contingent attributes. It is true that some of the ideas contain nothing of what we call sense perception. Even when sense perception is present our pleasure comes from some kind of uniformity, order, arrangement, or imitation and not from simple ideas of color or sound.

This seems to indicate that when teaching, education, or some prejudice gives rise to desire or repugnance in relation to some object, this desire or aversion is based on our conception of perfection or imperfection. Thus, if someone who is deprived of the sense of sight is affected by the desire for beauty, this desire must come from a feeling of regularity of form, of gracefulness in the voice, of tenderness, of softness, or some other quality that can only be perceived by senses other than sight, and without any relation to color.

Our philosophers only seem to take into account that pleasure of feeling which accompanies simple ideas of sensation. But there exist a great many agreeable feelings within those complex notions of objects which we call "beautiful" or "harmonious." It does not matter whether or not such notions of beauty and harmony are called "external sense perceptions" of sight and hearing. They should really be called an "internal sense" or an "internal feeling" if only to distinguish them from those other sensations of sight or hearing which can exist without any perception of beauty or harmony.

At this point we are faced with the question whether our senses can be a touchstone of truth. This would depend on the way in which we look at the senses. [4] When we want to present the most persuasive proof that something is true, we say that we have seen it with our own eyes; and if we really did see, one cannot fail but give credence to our assertion. In this case, the testimony of the senses is a first truth because it takes the place of the first principle without any need for looking further. Everyone agrees on that. On the other hand, it is also agreed that senses deceive, and experience shows that this cannot be doubted. Yet, if we are certain of the thing we have seen because we have seen it, how can the sense of sight deceive us, and if it can deceive us, how can we be sure of something simply because we saw it?

The common answer to this difficulty is that our sight and our senses can deceive us when they are not being exercised appropriately: the organ should be properly formed and the object should be at the right distance. But this really says nothing. What is the use of establishing rules to justify the testimony of our senses, on the basis of conditions which we could not ourselves defend, even though we know the circumstances under which they occur. What infallible rule can there be to judge whether the organ of my sight, my hearing, my sense of smell is actually properly adjusted? Our organs can only provide complete certitude when they are perfectly formed; this they are only in the case of perfect temperaments; and since these are extremely rare, it follows that there exists scarcely an organ that is not defective somewhere.

No matter how obvious this conclusion may seem, however, it does not destroy another truth, namely, that one is sure of what one sees. This contradiction shows that something has been left out, because a well-founded maxim cannot be in contradiction with another well-founded maxim. In order to develop this, let us examine in what way our senses are not a rule of truth, and how they are.

1. Our senses do not tell us what makes up that arrangement of bodies which is called "quality" and which creates certain impressions. Obviously I perceive that there is a certain arrangement in the body that makes me feel heat and weight; but what this arrangement is in itself ordinarily escapes my senses and often even my reason. I can perceive that with a certain arrangement and a certain movement in the smallest particles of this body, there exists a correlation between the body and the impression it makes upon me. Thus I conjecture that the sun's capacity to arouse in me a feeling of light consists in a certain movement or impulsion of little corpuscles through the pores of the air toward the retina of my eye; yet it is precisely this capacity that neither my eyes nor my reason can apprehend.

2. Our senses give no evidence at all of the infinite variety of combinations existing within the objects themselves which surpass the discernment of our sight, our hearing, our sense of smell. This can be clearly attested by the microscope; it has revealed to sight an infinite number of external combinations which indicate an equal variety in the internal parts. This complexity in turn produces a great variety of qualities. More perfect microscopes will reveal to us even further arrangements which at present we can neither perceive nor imagine.

3. The senses cannot teach us precisely what impressions other men receive through them. Such impressions depend upon the disposition of our organs which differ as widely as men's temperament or appearance; besides, the same external quality can produce different sensations in different men; we can see this every day. A certain liquid produces in me an unpleasant sensation and a pleasant one in someone else; therefore I cannot be sure that a certain body will produce upon someone else the exact same impression it made on me. Neither can I be certain that what I consider to be white does not appear as red to someone else.

4. Reason and experience teach us that bodies are in a continuous state of movement or change which is frequently quite imperceptible. We cannot know with any certainty whether a body has precisely the same quality from one day to the next, or the same capacity to produce an impression that it might have had previously. The body can alter and so can I. I may well be aware of a change in the impression produced, but the senses alone will not inform me whether this change is due to the object or to myself.

5. Our [5] senses do not permit us to judge either the absolute size of bodies or their absolute movement. The reason for this is obvious. Since our eyes differ, we cannot all have the same idea of the size of a body. We have to suppose that our eyes are merely natural spectacles and that the fluid in them produces the same effect as lenses do in spectacles, so that, according to the distance of the eyes from each other, according to the shape of the lens of the eye and its distance from the retina, we see objects differently. All this makes it impossible to affirm that there can exist two men who see the same size or the same details in objects, since it is impossible to state that their eyes are absolutely identical. Another natural consequence is that we cannot estimate the true or absolute length of movement of a body, but merely the relationship that these movements have to each other. It is an established fact that we can estimate the extent of the movement of a body only by the length of the distance this body has traveled. Thus, since our eyes cannot show us the real length of the distance, it follows that they cannot show us the full extent of the movement.

Let [6] us now examine what can be taken for first truths in the testimony of our senses. The first truths we receive from our senses can be reduced to three main categories:

(1) Our senses always communicate very faithfully the impression they receive. (2) These impressions almost always conform to truth in those matters which men need to know, unless there appears to be some reason for doubting them. (3) It is easy to judge when the testimony of the senses is doubtful on the following grounds:

1. The senses always communicate faithfully the impressions they receive. This is obvious because the senses are natural faculties dependent on the impressions they receive from objects. The relationships between the impression and what the senses communicate remain consistent. Someone on a fast moving ship will say that the shore moved in the opposite direction; it has to be this way, for the eye receives the same impression as it would if the shore line and the ship were actually moving in opposite directions; this is taught by observation in physics and by the rule of optics. From this point of view our senses never deceive us; it is our lack of caution which, in spite of their accurate report, causes us to deceive ourselves. Their accuracy does not consist in informing the soul of what exists but of what seems to be. It is up to the soul to discern the truth.

2. What appears to our senses is almost always consistent with truth, where the normal behavior and needs of life are concerned. Thus, for food, our senses make us sufficiently aware of the usual needs; in this way, the more something is to our advantage the greater ordinarily the number of sensations which permit us to recognize it; what we do not recognize with the help of the senses belongs not to our needs but to our curiosity.

3. The testimony [7] of the senses is infallible unless it is contradicted either by our reason or by a previous testimony of the same senses, either by the present testimony of another of our own senses, or by the testimony of another man's senses.

1. When our reason, relying on other sources, makes us take a decision in evident opposition to what appears to the senses, their testimony on this point cannot be a touchstone of truth. Thus, though the sun seems to be two feet in size and the stars one inch in diameter, reason, informed by incontestable facts and by obvious knowledge, teaches us that these heavenly bodies are infinitely larger than they seem to us.

2. When what appears now to our senses is different from what appeared to them previously: in this case we realize either that the object is no longer within reach, or that some change has occurred in the object or in our organ. In such cases one should decide not to judge at all rather than to make a wrong judgment.

Usage and experience help to distinguish the testimony of the senses. A child who perceives his image in water or in a mirror takes it for another child who is in the water or inside the mirror; but having experimented with putting his hand in the water or against the mirror, he will amend the sense of sight by means of the sense of touch and with time he will become convinced that there is no child where he thought he was seeing one. It can also happen that some Indian, coming from a country where water never freezes, will take a piece of ice for a stone; but after experience has shown that the piece of ice will turn to water, he amends the sense of touch by the sense of sight.

3. The third rule concerns the case when what appears to our senses is contrary to that which appears to the senses of other men, who are clearly constituted as we are. If my eyes show me something different from what appears to the eyes of all others, I have to believe that it is I who am in error rather than all the others; otherwise nature would be misleading a great number of men, which is not reasonable. See the Logic of Father Buffier, under the title of "First Truths."

This same author also states that some philosophers have been concerned with showing that our eyes constantly mislead us because they cannot properly show true size; but I would like to ask these philosophers if our eyes were given to us to judge the absolute size of objects. Everybody knows that the real function of the eyes is to observe color. It is true that accidentally, due to the different angles according to which rays of light strike the retina, the mind responds by making judgments about the distance and the size of objects. But that judgment is no more special to the sense of sight than it is to the sense of hearing. The latter also attests, almost accidentally, to the size and distance of resonant bodies, which cause more or less strong undulatory movements that make more or less of an impression on the ear. Would one have a serious proof that the senses are in error because the ear does not permit us to make a correct judgment of the size and distance of objects? It does not seem so. In such cases, after all, the ear is not really carrying out its proper function as organ of hearing but contributes by accident, as it were, to the sense of touch, which is the sense that should judge the size and distance of objects.

General usage will convince us of this. Size is measured in inches, feet, handbreadths, cubits, all of which are part of the human body. [8] In spite of the fact that the organ of touch is scattered over the entire body, it is nevertheless most noticeably located in the hand. It is therefore the hand that should rightly measure size by relating its own dimensions to the size of the object to which it is applied. Unless the testimony of the eyes is verified by the hand, any testimony by the eyes concerning size must be suspect. This does not render the sense of sight in any way more misleading or less perfect. It is in its nature to discern color and only incidentally the distance and size of objects. What good would it do to bring up here the example of the fly whose tiny eyes see objects in a totally different dimension than the eyes of an elephant? What conclusion can we draw: if the fly and the elephant had any intelligence, they would not have a false idea of size; for since size is relative, each would judge the dimension of an object according to its own size, which it can comprehend; they could say to themselves: "This object is so many times greater or smaller than my body, or a particular part of my body"; and in this way, in spite of the difference of their eyes, their judgment of size would always be true.

The same thing happens with men; no matter what impression the dimension of objects makes upon their eyes, all have an equally accurate idea of size because each measures it in relation to his proper size. [9]

We can say of our senses what we say of reason. When reason is thoughtfully directed, when it follows the natural insights God gave us, when it follows only the light of evidence and stops when ideas fail, then reason cannot mislead us. In the same way, the senses cannot mislead us when they act together, when they help each other and above all make use of experience. It is experience above all that cautions us against many of the errors that the senses by themselves could cause. Long usage alone teaches us to judge distances by sight and this, by examining, through touch, bodies we see, by observing bodies placed at different distances and in different ways, while being aware of the fact that the bodies themselves undergo no change.

All men have acquired this skill in early childhood; they constantly have to pay attention to the distance of objects; thus, imperceptibly, they learn to judge distance and later they persuade themselves that this is a gift of nature while it is actually the effect of long practice. The way in which vision is developed proves that the faculty of judging those objects we see is a skill acquired with usage and experience. If any doubt remains on this score it will be rapidly dispelled by the example of a young man born blind, who at the age of fourteen or so saw light for the first time. Here is the story as it is related by M. de Voltaire. [10]

If we add analogy to the testimony of the senses, we shall find in that another proof of the truth of things. Analogy has as its basis the very simple principle that the universe is governed by general and constant laws. By virtue of this we admit the following rule: Similar effects have the same causes.

The usefulness of analogy consists in the fact that it saves us thousands of useless discussions which we would have to repeat for each particular subject. It is sufficient for us to know that everything is ruled according to general laws in order to have a justification for believing that bodies which appear similar to us have the same properties, that the fruit from the same tree all has similar taste, etc. The certainty that accompanies analogy extends to the senses, which in their turn are the basis of all reasoning by analogy.

Speaking of knowledge, we have said that without the help of the senses, man could acquire no knowledge of bodily objects; but at the same time we have noted that the senses alone were not sufficient, since there exists not a single man capable of examining by himself all those things necessary for living. This means that in an infinite number of cases men have to learn from each other and have to communicate their observations to each other; otherwise they could not benefit in any way from most of the things that God has granted them. This leads us to conclude that it is God's will that under certain circumstances the testimony of the senses be another sign of truth. But, if in certain cases the testimony is infallible, the senses must be equally so, since the testimony is based on the senses . Thus, if we prove that in certain cases the testimony of men is a reliable rule for truth, this proves the same thing for the senses on which this testimony is necessarily based.

Notes

1. [J. G. Barker, in his Diderot's Treatment of the Christian Religion in the Encyclopédie (King's Crown Press: New York, 1941), has traced many articles to their origins in Father Buffier's writings.]

2. [Proust, Diderot et l'Encyclopédie (Paris: Armand Colin, 1962), pp. 157–159, esp. p. 157, n. 171 and n. 180.]

3. [Locke, see especially in An Essay on the Human Understanding , Bk. II, Chaps. I and XIX.]

4. [Here begins the almost word-for-word copying from Buffier's Traité des Premières Verités , Chaps. XIV, XV, and XVII. See F. Bouillier (ed.), Œuvres Philosophiques du Père Buffier (Paris: Charpentier, 1843).]

5. [This paragraph is not from Father Buffier.]

6. [Here the text from Buffier is picked up once again.]

7. [This paragraph is not in Buffier. The text resumes with 1, following.]

8. [The French words are pouces ("thumbs"), pieds ("feet"), mains ("hands"), coudées ("elbow lengths").]

9. [Here ends the text from Buffier.]

10. [Voltaire, Élémens (sic) de la Philosophie de Newton , in Œuvres de Voltaire (Paris: P. Pourrat Frères, 1839), pp. 177–178. This work appeared originally in 1738. The same example is used by Condillac in his Treatise on the Sensations .]