M. Tullius Cicero, De Officiis
Walter Miller, Ed.

[19] For it often happens, owing to exceptional circumstances, that what is accustomed under ordinary circumstances to be considered morally wrong is found not to be morally wrong. For the sake of illustration, let us assume some particular case that admits of wider application: what more atrocious crime can there be than to kill a fellow-man, and especially an intimate friend? But if anyone kills a tyrant—be he never so intimate a friend—he has not laden his soul with guilt, has he? The Roman People, at all events, are not of that opinion; for of all glorious deeds they hold such an one to be the most noble. Has expediency, then, prevailed over moral rectitude? Not at all; moral rectitude has gone hand in hand with expediency.

Some general rule, therefore, should be laid down¹ to enable us to decide without error, whenever what we call the expedient seems to clash with what we feel to be morally right; and, if we follow that rule in comparing courses of conduct, we shall never swerve from the path of duty.