Sin is contrary to virtue, not by reason of itself, but by reason of its act.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic virtue sin
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

“As to the acquired virtues, they are not destroyed by one act of any kind of sin. Accordingly, mortal sin is incompatible with the infused virtues, but is consistent with acquired virtue: while venial sin is compatible with virtues, whether infused or acquired. Reply Obj. 1: Sin is contrary to virtue, not by reason of itself, but by reason of its act. Hence sin is incompatible with the act, but not with the habit, of virtue. Reply Obj. 2: Vice is directly contrary to virtue, even as sin to virtuous act: and so vice excludes virtue, just as sin excludes acts of virtue.” source