Objection 1: It would seem that use is to be found in irrational animals. For it is better to enjoy than to use, because, as Augustine says (De Trin. x, 10): "We use things by referring them to something else which we are to enjoy." But enjoyment is to be found in irrational animals, as stated above (Q. 11, A. 2). Much more, therefore, is it possible for them to use.
Obj. 2: Further, to apply the members to action is to use them. But irrational animals apply their members to action; for instance, their feet, to walk; their horns, to strike. Therefore it is possible for irrational animals to use.
On the contrary, Augustine says (QQ. 83, qu. 30): "None but a rational animal can make use of a thing."
I answer that, as stated above (A. 1), to use is to apply an active principle to action: thus to consent is to apply the appetitive movement to the desire of something, as stated above (Q. 15, AA. 1, 2, 3). Now he alone who has the disposal of a thing, can apply it to something else; and this belongs to him alone who knows how to refer it to something else, which is an act of the reason. And therefore none but a rational animal consents and uses.
Reply Obj. 1: To enjoy implies the absolute movement of the appetite to the appetible: whereas to use implies a movement of the appetite to something as directed to something else. If therefore we compare use and enjoyment in respect of their objects, enjoyment is better than use; because that which is appetible absolutely is better than that which is appetible only as directed to something else. But if we compare them in respect of the apprehensive power that precedes them, greater excellence is required on the part of use: because to direct one thing to another is an act of reason; whereas to apprehend something absolutely is within the competency even of sense.
Reply Obj. 2: Animals by means of their members do something from natural instinct; not through knowing the relation of their members to these operations. Wherefore, properly speaking, they do not apply their members to action, nor do they use them.