But evil acts, for it corrupts good. Therefore evil is a being and a nature.
 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

“But evil and good are not opposed as privation and habit, but as contraries, as the Philosopher shows (Praedic. x) by the fact that between good and evil there is a medium, and from evil there can be a return to good. Therefore evil signifies a nature.
Obj. 4: Further, what is not, acts not. But evil acts, for it corrupts good. Therefore evil is a being and a nature. Obj. 5: Further, nothing belongs to the perfection of the universe except what is a being and a nature. But evil belongs to the perfection of the universe of things; for Augustine says (Enchir. 10, 11) that the "admirable beauty of the universe is made up of all things.” source