The loss itself of good is apprehended as an evil, just as the loss of evil is apprehended as a good
 Thomas Aquinas, Summa Theologica (1274). copy citation

add
Author Thomas Aquinas
Source Summa Theologica
Topic loss evil
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

“On the other hand love, which is the cause of pleasure and sorrow, regards good rather than evil: and therefore, forasmuch as the object is the cause of a passion, the present evil is more properly the cause of sorrow or pain, than the good which is lost. Reply Obj. 1: The loss itself of good is apprehended as an evil, just as the loss of evil is apprehended as a good: and in this sense Augustine says that pain results from the loss of temporal goods. Reply Obj. 2: Pleasure and its contrary pain have the same object, but under contrary aspects: because if the presence of a particular thing be the object of pleasure, the absence of that same thing is the object of sorrow.” source