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.