Now love of self is a good and right thing in itself: wherefore man is commanded to love his neighbor as himself (Lev. 19:18). Therefore self-love cannot be the proper cause of 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

“FOURTH ARTICLE [I-II, Q. 77, Art. 4]
Whether Self-love Is the Source of Every Sin?
Objection 1: It would seem that self-love is not the source of every sin. For that which is good and right in itself is not the proper cause of sin. Now love of self is a good and right thing in itself: wherefore man is commanded to love his neighbor as himself (Lev. 19:18). Therefore self-love cannot be the proper cause of sin. Obj. 2: Further, the Apostle says (Rom. 7:8) : "Sin taking occasion by the commandment wrought in me all manner of concupiscence"; on which words a gloss says that "the law is good, since by forbidding concupiscence, it forbids all evils," the reason for which is that concupiscence is the cause of every sin.” source