But because good in itself is the object of the will, evil, which is the privation of good, is found in a special way in rational creatures which have a will.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic evil good
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
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Context

“In one way it occurs by the subtraction of the form, or of any part required for the integrity of the thing, as blindness is an evil, as also it is an evil to be wanting in any member of the body. In another way evil exists by the withdrawal of the due operation, either because it does not exist, or because it has not its due mode and order. But because good in itself is the object of the will, evil, which is the privation of good, is found in a special way in rational creatures which have a will. Therefore the evil which comes from the withdrawal of the form and integrity of the thing, has the nature of a pain; and especially so on the supposition that all things are subject to divine providence and justice, as was shown above (Q. 22, A. 2) ; for it is of the very nature of a pain to be against the will.” source