As all our knowledge turns upon the relations by which one object differs from another, if there existed no brute animals, the nature of the human being would be still more incomprehensible. Having considered man in himself, ought we not to derive every assistance, by comparing him with the other parts of the animal creation? We will proceed then to examine the nature of animals, to compare their organization, to study their general economy, thereby to make particular applications, to mark resemblances, to reconcile the differences; and from the assemblage of those combinations, to distinguish the principal effects of the living mechanism, and to make a further progress in that important knowledge of which man is the object. We will begin by reducing within its proper limits a subject which, at first view, appears to be immense. The properties of matter which animals possess in common with inanimate beings come not within our present consideration, and which we have already fully treated upon. For the same reason we shall reject such qualities as are found equally to belong to the vegetable and to the animal. As in the class of animals we comprehend a number of animated beings, whose organization is highly different from that of man, as well as from more perfect animals, so we shall wave the consideration of them, and confine ourselves to those animals which have evidently the greatest affinity to us. But as the nature of man is superior to that of animals, so of that superiority we shall study to demonstrate the cause, in order that we may distinguish what is peculiar to man, from what belongs to him in common with other animals. Previous to an examination of the minute parts of the animal machine, and their peculiar functions, let us view the general result of this mechanism, and, without at first reasoning upon causes, confine ourselves to an elucidation and description of effects. An animal has two modes of existence; that of motion, or awake, and rest, or asleep; and which, while life lasts, succeed each other alternately. In the former, all the springs of the machine are in action; in the latter, there is only a part of them so, and this part acts as well while the animal is asleep as while it is awake, and is therefore absolutely necessary since the animal cannot exist without it. It is also independent of the other, as it acts of itself; the former, on the contrary, depends on the latter, as it cannot exercise itself alone. The one is a fundamental part of the animal economy, since it acts continually and without interruption; the other is less essential, since it acts but by internals.