Evil is said to be outside the will, because the will does not tend to it under the aspect of evil.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic evil
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

“, it follows that the will, which is the principle of voluntary acts, both of good acts, and of evil acts or sins, is the principle of sins. Therefore it follows that sin is in the will as its subject. Reply Obj. 1: Evil is said to be outside the will, because the will does not tend to it under the aspect of evil. But since some evil is an apparent good, the will sometimes desires an evil, and in this sense is in the will. Reply Obj. 2: If the defect in the apprehensive power were nowise subject to the will, there would be no sin, either in the will, or in the apprehensive power, as in the case of those whose ignorance is invincible.” source