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?