For perfect and true happiness requires that one should be certain of being happy for ever, else the will would not rest.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic happiness rest
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/18755/pg18755-images.html

Context

“Now that which is not known can neither be restful nor repugnant to the will: wherefore Augustine says (Gen. ad lit. xi, 17) that the angels could not be perfectly happy in their first state before their confirmation, or unhappy before their fall, since they had no foreknowledge of what would happen to them. For perfect and true happiness requires that one should be certain of being happy for ever, else the will would not rest. In like manner, since the everlastingness of damnation is a necessary condition of the punishment of the damned, it would not be truly penal unless it went against the will; and this would be impossible if they were ignorant of the everlastingness of their damnation.” source