Thomas Aquinas

“ On account of the diversity of objects subject to the act of the reason, nothing prevents the reason from participating in itself: thus the knowledge of principles is participated in the knowledge of the conclusions. ”

“ Impulse to action is in irrational animals otherwise than in man. For the impulse of man to action arises from the directing reason; wherefore his impulse is one of command. On the other hand, the impulse of the irrational animal arises from natural instinct; because as soon as they apprehend the fitting or the unfitting, their appetite is moved naturally to pursue or to avoid. Wherefore they are directed by another to act; and they themselves do not direct themselves to act. Consequently in them is impulse but not command. ”

“ I answer that, To command is nothing else than to direct someone to do something, by a certain motion of intimation. Now to direct is the proper act of reason. Wherefore it is impossible that irrational animals should command in any way, since they are devoid of reason.
Reply to Objection 1: The appetitive power is said to command movement, in so far as it moves the commanding reason. But this is only in man. In irrational animals the appetitive power is not, properly speaking, a commanding faculty, unless command be taken loosely for motion. ”

“ The body of the irrational animal is competent to obey; but its soul is not competent to command, because it is not competent to direct. Consequently there is no ratio there of commander and commanded ”

“ But those things which are distinct in substance, and one according to an accident, are distinct simply, and one in a certain respect: thus many men are one people, and many stones are one heap; which is unity of composition or order. In like manner also many individuals that are one in genus or species are many simply, and one in a certain respect: since to be one in genus or species is to be one according to the consideration of the reason. ”

“ This is the case with these two organs in particular, because each is as it were a separate animal being, in so far as it is a principle of life; and the principle is virtually the whole. For the heart is the principle of the senses; and from the organ of generation proceeds the seminal virtue, which is virtually the entire animal. ”

“ Now the reason can intimate or declare something in two ways. First, absolutely: and this intimation is expressed by a verb in the indicative mood, as when one person says to another: "This is what you should do." Sometimes, however, the reason intimates something to a man by moving him thereto; and this intimation is expressed by a verb in the imperative mood ”