He must be able to hear them [the counter arguments] from persons who actually believe them; who defend them in earnest, and do their very utmost for them. He must know them in their most plausible and persuasive form; he must feel the whole force of the difficulty which the true view of the subject has to encounter and dispose of; else he will never really possess himself of the portion of truth which meets and removes that difficulty.
John Stuart MillThere is a very real evil consequent on ascribing supernatural origin to the received maxilms of morality. That origin consecrates the whole of them and protects them from being discussed or criticized.
John Stuart Mill