It is evident, therefore, that in brutes the interior sense differs in nothing from the exterior but in the property of retaining the impressions it has received, a property by which alone all the actions of animals may be explained, and some idea obtained of what passes within them; a property which likewise demonstrates the essential and infinite difference which subsists between them and us, and from which may be distinguished in what respects they are similar.
The degrees of excellence in the senses do not follow the same order in the brute as in the human species. The sense which has the strongest affinity to thought, is the touch. This is enjoyed by man in greater perfection than by animals. That which has the strongest affinity to instinct and appetite, is that of smelling; a sense in which man must acknowledge an infinite inferiority. Man, then, has the greatest tendency to knowledge, and the brute to appetite. In the former, the sense first in point of excellence, is the touch, and smelling the last; and this difference corresponds with the nature of each. The sense of seeing is at best uncertain, without the aid of the touch, and therefore less capable of perfection in the brute than in man. The ear, though perhaps as perfect in the former as in the latter, is of much less use to the animal, from the want of speech, which in man is an appendage to the sense of hearing, an organ of communication which renders it an active sense; whereas in the other hearing is a sense almost entirely passive. Man, then, enjoys the senses of feeling, seeing, and hearing, more perfect, and the sense of smelling more imperfectly than other animals; and as the taste is an inferior smell, and has also a stronger relation to appetite than any of the other senses, there is a sufficient probability to suppose that animals enjoy it in a more exquisite degree than man. Of this a proof might be adduced from the repugnance which animals have to certain kinds of food, and from their natural appetite for such as are proper for them; while man, unless informed of the difference, would eat the fruit of one tree for that of another, and even hemlock for parsley.
The excellence of the senses proceeds from Nature; but art and habit may render them still more perfect. A painter sees, at the first glance, numbers of shades and differences, which another person will pass over unnoticed. A musician, always habituated to harmony, receives a lively sensation of pain from discord. In like manner are the senses, and even appetites of animals rendered more perfect. Birds may be taught to repeat words, and imitate tunes; and the ardour of a dog for the chace may be increased by accustoming him to a certain reward.
In proportion as these senses are acute and perfect does the animal shew itself active and intelligent. In man the improvement is not so conspicuous, because he exercises his ear and his eye by means more rational and ingenious. Those persons who see, hear, or smell, imperfectly, are of no less intellectual capacity than others; an evident proof that in man there is something more than an internal animal sense. This is the soul of man, which is a superior sense, a spiritual substance, entirely different in its essence and action from the nature of the external senses.
From this, however, we are not to deny that there is in man an internal material sense corresponding with the external senses. But what I maintain is, that the latter is infinitely subordinate to the other; that the spiritual substance governs it, and either destroys or creates its operations. In the animal this sense is the determinating principle of motion, but in man only the means, or the secondary cause.
Let us endeavour to clear up this important point, and let us see what power this internal material sense possesses, and what it is capable of producing. The internal material sense receives promiscuously all the impressions the external senses transmit to it. These impressions proceed from the action of objects; they only pass over the external senses, and produce in them but an instantaneous vibration; they rest, however, upon the internal sense, and produce in the brain, which is its organ, durable and distinct agitations. These vibrations create appetite or disgust, inclination or repugnance, according to the present state and disposition of an animal. An animal, the instant after its birth, begins to breathe, and to feel the want of nourishment; the smell, which is the sense of appetite, receives the emanations of the milk which is contained in the teats of its mother.
The vibrations which this sense undergoes, from the odoriferous particles, are communicated to the brain, which acting, in its turn, upon the nerves, the animal is stimulated to open its mouth, to obtain that sustenance of which it feels the want. The sense of appetite being less acute in man than in brutes, the infant at its birth feels only the desire of receiving nourishment, which it announces by its cries, but it cannot obtain it of itself; it receives no information from the smell, and is obliged to have its mouth put to the nipple, when the agitations, excited by the touch and smell, are communicated to the brain and nerves, and the child makes the necessary motions for sucking in its nourishment. Solely by the smell and taste, the senses of appetite, can the animal be informed of the presence of its food, and of the place where it is, as its eyes are still closed, and would, even if they were open, in no degree contribute towards the determination of motion. Vision has a greater relation to knowledge than to appetite, and in man the eye is open from the moment of his birth; in most animals it is shut for several days, but in whom the senses of appetite are far more expanded, and more perfect.
The same remark is alike applicable to progressive motion, and to all the other exterior movements. A new-born infant can hardly move its members, and it is a long time before it attains strength sufficient to change its place, but in a very little time does a young animal acquire these faculties. In the animal these powers relate solely to the appetite, which is vehement, quickly developed, and the sole principle of motion; in man the appetite is weak, more slowly developed, and can have less influence than knowledge upon the determination of motion; man is necessarily, in this respect, more backward than the animal.
Every thing concurs then to prove, even in a physical sense, that brutes are actuated by appetite alone, and that man is governed by a superior principle. If doubts still exist, it is from our imperfect conception how appetite alone is capable of producing, in animals, effects so much resembling those which knowledge produces among ourselves; and from the difficulty we have to distinguish what we do in virtue of knowledge, from what we do by the mere force of appetite. Yet, in my opinion, it is not impossible to dispel this uncertainty. The internal material sense retains for a long time the agitations it receives; it is a sense of which the brain is the organ, and by which all the impressions are received that each of the exterior senses transmits to it. When, therefore, an exterior impression proceeds from the senses of appetite, the animal will advance to attain, or draw back to avoid, the object of this impression. This motion, however, is liable to uncertainty when produced by the eye or the ear; because, when an animal sees, or hears, for the first time, he will be agitated by light or by sound; yet this agitation will be uncertain, since neither have any relation to appetite. It is only by repeated acts of seeing and hearing, added to the senses of taste and feeling, that it will actually advance or recede from objects which become relative to its appetite. A dog, for instance, who has been tutored, however violent his appetite, will not seize what might satisfy that appetite, although he will use every gesture to obtain it from the hand of its master. Does not this animal seem to reason between desire and fear, nearly as a man would do, who was inclined to seize upon the property of another, but was withheld by the dread of punishment? Though this analogy may be just; yet to render it in effect well-founded, should not animals be capable of performing the same actions that we perform? Now the contrary is evident; as nothing do animals either invent or perfect; in every thing they have an uniformity, and consequently no reflection. Of this analogy then we may doubt its reality, and may with propriety enquire, whether it is not by a principle different from ours that brutes are directed? and whether, without being under the necessity of allowing them the aid of reflection, the senses they enjoy are not sufficient to produce the actions they perform?
Whatever relates to their appetites strongly agitates their interior sense; and on the object of this appetite the dog would instantly rush, did not this very sense retain the impressions of pain which had formerly accompanied this action. By exterior impressions the animal has been modified. This prey is not presented to a dog simply, but to one which has been chastised every time it obeyed this impulse of appetite; the agitations of pain, therefore, are renewed when those of appetite are felt, having been constantly felt at the same time. The animal being thus impelled at once by two contrary powers, two powers destructive of each other, remains between them in an equilibrium; and, as the determinate cause of its motion is counterbalanced, it makes no effort to attain the object of its appetite. Though the agitations of appetite and repugnance, or of pleasure and pain, destroy the effect of each other, in the brain a third vibration takes place, which accompanies the other two, and this is occasioned by the action of its master, from whose hand the animal has often received its food; and as this is in no degree opposed or counterbalanced, it becomes the determinative cause of motion; and the dog is therefore determined to move towards its master, and to remain in motion till its appetite is entirely satisfied.