Uti proprie est actus voluntatis, quia voluntas est quae movet potentias animae ad suos actus, et hoc est applicare eas ad operationem.
Use is, properly speaking, an act of the will, because it is the will which moves the soul's powers to their acts, and this is to apply them to operation.
Ratio quidem in aliud refert, sed voluntas tendit in id quod est in aliud relatum per rationem. Et secundum hoc dicitur quod uti est referre aliquid in alterum.
Reason does indeed refer one thing to another, but the will tends to that which is referred by the reason to something else. And in this sense to use is to refer one thing to another.
Uti primo et principaliter est voluntatis, tanquam primi moventis; rationis autem tanquam dirigentis; sed aliarum potentiarum tanquam exequentium, quae comparantur ad voluntatem, a qua applicantur ad agendum, sicut instrumenta ad principale agens.
First and principally, use belongs to the will as first mover; to the reason, as directing; and to the other powers as executing the operation, which powers are compared to the will which applies them to act, as the instruments are compared to the principal agent.
Actio autem proprie non attribuitur instrumento, sed principali agenti, sicut aedificatio aedificatori, non autem instrumentis.
Now action is properly ascribed, not to the instrument, but to the principal agent, as building is ascribed to the builder, not to his tools.
Etiam ipsa ratio speculativa applicatur ad opus intelligendi vel iudicandi, a voluntate. Et ideo intellectus speculativus uti dicitur tanquam a voluntate motus, sicut aliae executivae potentiae.
Even the speculative reason is applied by the will to the act of understanding or judging. Consequently the speculative reason is said to use, insofar as it is moved by the will, in the same way as the other executing powers.