A man sins by his will, not only when he wills an evil end; but also when he wills an evil act.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic evil 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

“and if the will be evil either by reason of its intention of the end, or by reason of the act willed, it follows that the external action is evil. Reply Obj. 1: If the good tree be taken to signify the good will, it must be in so far as the will derives goodness from the act willed and from the end intended. Reply Obj. 2: A man sins by his will, not only when he wills an evil end; but also when he wills an evil act. Reply Obj. 3: Voluntariness applies not only to the interior act of the will, but also to external actions, inasmuch as they proceed from the will and the reason. Consequently the difference of good and evil is applicable to both the interior and external act.” source