But the evil of sin cannot be in the good of virtue or of grace, because they are contrary to it. Therefore it must be in the good of nature, and consequently it does not destroy it entirely.
 Thomas Aquinas, Summa Theologica (1274). copy citation

add
Author Thomas Aquinas
Source Summa Theologica
Topic evil virtue
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17897/pg17897-images.html

Context

“Now this aptitude is destroyed entirely in some on account of sin: thus the lost cannot be restored to virtue any more than the blind can to sight. Therefore sin can take away the good of nature entirely.
On the contrary, Augustine says (Enchiridion xiv) that "evil does not exist except in some good." But the evil of sin cannot be in the good of virtue or of grace, because they are contrary to it. Therefore it must be in the good of nature, and consequently it does not destroy it entirely. I answer that, As stated above (A. 1) , the good of nature, that is diminished by sin, is the natural inclination to virtue, which is befitting to man from the very fact that he is a rational being; for it is due to this that he performs actions in accord with reason, which is to act virtuously.” source