Now the fact that anyone desires a temporal good inordinately, is due to the fact that he loves himself inordinately; for to wish anyone some good is to love him. Therefore it is evident that inordinate love of self is the cause of every sin.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic sin love
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

“I answer that, As stated above (Q. 75, A. 1) , the proper and direct cause of sin is to be considered on the part of the adherence to a mutable good; in which respect every sinful act proceeds from inordinate desire for some temporal good. Now the fact that anyone desires a temporal good inordinately, is due to the fact that he loves himself inordinately; for to wish anyone some good is to love him. Therefore it is evident that inordinate love of self is the cause of every sin. Reply Obj. 1: Well ordered self-love, whereby man desires a fitting good for himself, is right and natural; but it is inordinate self-love, leading to contempt of God, that Augustine (De Civ. Dei xiv, 28) reckons to be the cause of sin.
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