Further, evil is not a being; whereas good is a being.
 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
Weblink http://www.gutenberg.org/cache/epub/17611/pg17611-images.html

Context

“It would seem that evil is not in good as its subject. For good is something that exists. But Dionysius says (Div. Nom. iv, 4) that "evil does not exist, nor is it in that which exists." Therefore, evil is not in good as its subject. Obj. 2: Further, evil is not a being; whereas good is a being. But "non-being" does not require being as its subject. Therefore, neither does evil require good as its subject. Obj. 3: Further, one contrary is not the subject of another. But good and evil are contraries.” source