In like manner if a man's reason or conscience tell him, that what is indifferent in itself, for instance to raise a straw from the ground, is forbidden or commanded, his reason or conscience errs.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic conscience reason
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

“And they say that if reason or conscience tell us to do something which is good generically, there is no error: and in like manner if it tell us not to do something which is evil generically; since it is the same reason that prescribes what is good and forbids what is evil. On the other hand if a man's reason or conscience tells him that he is bound by precept to do what is evil in itself; or that what is good in itself, is forbidden, then his reason or conscience errs. In like manner if a man's reason or conscience tell him, that what is indifferent in itself, for instance to raise a straw from the ground, is forbidden or commanded, his reason or conscience errs. They say, therefore, that reason or conscience when erring in matters of indifference, either by commanding or by forbidding them, binds: so that the will which is at variance with that erring reason is evil and sinful.” source