But things which are evil among us are evil not essentially, but by participation. Therefore we must seek for some supreme essential evil, which is the cause of every evil.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic evil participation
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 good and better are so considered in relation to what is best. Therefore evil and worse are so considered in relation to some supreme evil.
Obj. 4: Further, everything participated is reduced to what is essential. But things which are evil among us are evil not essentially, but by participation. Therefore we must seek for some supreme essential evil, which is the cause of every evil. Obj. 5: Further, whatever is accidental is reduced to that which is per se. But good is the accidental cause of evil. Therefore, we must suppose some supreme evil which is the per se cause of evils. Nor can it be said that evil has no per se cause, but only an accidental cause; for it would then follow that evil would not exist in the many, but only in the few.
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