A work can be toilsome and difficult in two ways: first, from the greatness of the work, and thus the greatness of the work pertains to the increase of merit
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic greatness merit
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

“Charity, inasmuch as it has the last end for object, moves the other virtues to act. For the habit to which the end pertains always commands the habits to which the means pertain, as was said above (Q. 9, A. 1) . Reply Obj. 2: A work can be toilsome and difficult in two ways: first, from the greatness of the work, and thus the greatness of the work pertains to the increase of merit; and thus charity does not lessen the toil—rather, it makes us undertake the greatest toils, "for it does great things, if it exists," as Gregory says (Hom. in Evang. xxx) . Secondly, from the defect of the operator;” source