Wherefore those things that are naturally proportionate to a certain end, are said to desire that end naturally. Yet to have an end thus is to have it imperfectly. Now every imperfect thing tends to perfection.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic perfection desire
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 contrary, Damascene says (De Fide Orth. ii, 22) that "the will after choosing has an impulse to the operation, and afterwards it uses (the powers) ." Therefore use follows choice.
I answer that, The will has a twofold relation to the thing willed. One, according as the thing willed is, in a way, in the willing subject, by a kind of proportion or order to the thing willed. Wherefore those things that are naturally proportionate to a certain end, are said to desire that end naturally. Yet to have an end thus is to have it imperfectly. Now every imperfect thing tends to perfection. And therefore both the natural and the voluntary appetite tend to have the end in reality; and this is to have it perfectly. This is the second relation of the will to the thing willed.
Now the thing willed is not only the end, but also the means.” source