THE TWOFOLD PRINCIPLE OF ACTIVITY AND THE POSSIBILITY OF DEFECT THEREIN
There are certain actions whose principle is not nature but the will. The object of the will is the good, which consists primarily in the end, secondarily in whatever leads to the end. Voluntary action is related to the good as natural action is related to the form by which a thing acts. Consequently, just as a defect in natural activity cannot ensue in things that do not admit of defect in their forms, but can occur only in corruptible things whose forms are defectible, so voluntary actions can be deficient only in beings whose will can deflect from their proper end. Hence, if the will cannot deflect from its proper end, deficiency in voluntary action is clearly impossible.
The will cannot be deficient with regard to the good which is the very nature of the being that wills; for every being seeks in its own way its perfection, which is each one's good. But as regards an external good the will can be deficient, by resting content with a good connatural to it. Therefore if the nature of the being that wills is the ultimate end of its will, no deficiency in voluntary action can arise.
Such is the case with God alone. For His goodness, which is the ultimate end of things, is His very nature. But the nature of other beings endowed with will is not the ultimate end of their will. Hence a defect in voluntary action can occur in them, if their will remains fixed on their own good and does not push on to the supreme good, which is the last end. Therefore in all created intellectual substances a deficiency in voluntary action is possible.