I answer that, It is impossible for any sorrow or pain to be man's greatest evil. For all sorrow or pain is either for something that is truly evil, or for something that is apparently evil, but good in reality.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic reality pain
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

“For Augustine says (De Vera Relig. xii) : "What is pain of the soul, except for the soul to be deprived of that which it was wont to enjoy, or had hoped to enjoy? And this is all that is called evil, i.e. sin, and the punishment of sin." Therefore sorrow or pain is not man's greatest evil.
I answer that, It is impossible for any sorrow or pain to be man's greatest evil. For all sorrow or pain is either for something that is truly evil, or for something that is apparently evil, but good in reality. Now pain or sorrow for that which is truly evil cannot be the greatest evil: for there is something worse, namely, either not to reckon as evil that which is really evil, or not to reject it. Again, sorrow or pain, for that which is apparently evil, but really good, cannot be the greatest evil, for it would be worse to be altogether separated from that which is truly good.” source