The virtue of a cause is measured by the effect that flows from the nature of the cause, not by that which results by accident.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic virtue accident
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 other hand, if the consequences follow by accident and seldom, then they do not increase the goodness or malice of the action: because we do not judge of a thing according to that which belongs to it by accident, but only according to that which belongs to it of itself. Reply Obj. 1: The virtue of a cause is measured by the effect that flows from the nature of the cause, not by that which results by accident. Reply Obj. 2: The good actions done by the hearers, result from the preacher's words, as an effect that flows from their very nature. Hence they redound to the merit of the preacher: especially when such is his intention.” source