It is impossible for man to make up his mind to commit a sin, unless that mental faculty which has the sovereign power of urging his members to, or restraining them from, act, yield to the evil deed and become its slave.
 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

“But man often consents to an act not for eternal, but for temporal reasons, or even on account of some passion of the soul. Therefore consent to an act does not belong to the higher reason alone. On the contrary, Augustine says (De Trin. xii, 12) : "It is impossible for man to make up his mind to commit a sin, unless that mental faculty which has the sovereign power of urging his members to, or restraining them from, act, yield to the evil deed and become its slave." I answer that, The final decision belongs to him who holds the highest place, and to whom it belongs to judge of the others; for as long as judgment about some matter remains to be pronounced, the final decision has not been given.” source