Objection 1. It would seem that sin does not diminish the good of nature. For man's sin is no worse than the devil's. But natural good remains unimpaired in devils after sin, as Dionysius states (Div. Nom. iv). Therefore neither does sin diminish the good of human nature.
Objection 2. Further, when that which follows is changed, that which precedes remains unchanged, since substance remains the same when its accidents are changed. But nature exists before the voluntary action. Therefore, when sin has caused a disorder in a voluntary act, nature is not changed on that account, so that the good of nature be diminished.
Objection 3. Further, sin is an action, while diminution is a passion. Now no agent is passive by the very reason of its acting, although it is possible for it to act on one thing, and to be passive as regards another. Therefore he who sins, does not, by his sin, diminish the good of his nature.
Objection 4. Further, no accident acts on its subject: because that which is patient is a potential being, while that which is subjected to an accident, is already an actual being as regards that accident. But sin is in the good of nature as an accident in a subject. Therefore sin does not diminish the good of nature, since to diminish is to act.
On the contrary, "A certain man going down from Jerusalem to Jericho (Luke 10:30), i.e. to the corruption of sin, was stripped of his gifts, and wounded in his nature," as Bede [The quotation is from the Glossa Ordinaria of Strabo] expounds the passage. Therefore sin diminishes the good of nature.
I answer that, The good of human nature is threefold. First, there are the principles of which nature is constituted, and the properties that flow from them, such as the powers of the soul, and so forth. Secondly, since man has from nature an inclination to virtue, as stated above (60, 1; 63, 1), this inclination to virtue is a good of nature. Thirdly, the gift of original justice, conferred on the whole of human nature in the person of the first man, may be called a good of nature.
Accordingly, the first-mentioned good of nature is neither destroyed nor diminished by sin. The third good of nature was entirely destroyed through the sin of our first parent. But the second good of nature, viz. the natural inclination to virtue, is diminished by sin. Because human acts produce an inclination to like acts, as stated above (Question 50, Article 1). Now from the very fact that thing becomes inclined to one of two contraries, its inclination to the other contrary must needs be diminished. Wherefore as sin is opposed to virtue, from the very fact that a man sins, there results a diminution of that good of nature, which is the inclination to virtue.
Reply to Objection 1. Dionysius is speaking of the first-mentioned good of nature, which consists in "being, living and understanding," as anyone may see who reads the context.
Reply to Objection 2. Although nature precedes the voluntary action, it has an inclination to a certain voluntary action. Wherefore nature is not changed in itself, through a change in the voluntary action: it is the inclination that is changed in so far as it is directed to its term.
Reply to Objection 3. A voluntary action proceeds from various powers, active and passive. The result is that through voluntary actions something is caused or taken away in the man who acts, as we have stated when treating of the production of habits (51, 2).
Reply to Objection 4. An accident does not act effectively on its subject, but it acts on it formally, in the same sense as when we say that whiteness makes a thing white. In this way there is nothing to hinder sin from diminishing the good of nature; but only in so far as sin is itself a diminution of the good of nature, through being an inordinateness of action. But as regards the inordinateness of the agent, we must say that such like inordinateness is caused by the fact that in the acts of the soul, there is an active, and a passive element: thus the sensible object moves the sensitive appetite, and the sensitive appetite inclines the reason and will, as stated above (77, A1, 2). The result of this is the inordinateness, not as though an accident acted on its own subject, but in so far as the object acts on the power, and one power acts on another and puts it out of order.
Source: Summa Theologica (New Advent)