Now not all sins are evil through being forbidden, but some are forbidden because they are evil. Therefore sin in general should not be defined as being against the law of God.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic evil sin
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

“Now the definition in question contains no mention of aversion from our due end: therefore it is an insufficient definition of sin.
Obj. 4: Further, a thing is said to be forbidden, because it is contrary to the law. Now not all sins are evil through being forbidden, but some are forbidden because they are evil. Therefore sin in general should not be defined as being against the law of God. Obj. 5: Further, a sin denotes a bad human act, as was explained above (A. 1). Now man's evil is to be against reason, as Dionysius states (Div. Nom. iv) . Therefore it would have been better to say that sin is against reason than to say that it is contrary to the eternal law.
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