He must be able to hear them ...

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.
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 Mill

Quotes from the same author

Accordingly, France Had Voltaire, and his school of negative thinkers, and England (or rather Scotland) had the profoundest negative thinker on record, David Hume: a man, the peculiarities of whose mind qualified him to detect failure of proof, and want of logical consistency, at a depth which French skeptics, with their comparatively feeble powers of analysis and abstractions stop far short of, and which German subtlety alone could thoroughly appreciate, or hope to rival.
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 John Stuart Mill
What citizens of a free country would listen to any offers of good and skillful administration in return for the abdication of freedom?
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 John Stuart Mill
Men and governments must act to the best of their ability. There is no such thing as absolute certainty but there is assurance sufficient for the purposes of human life.
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 John Stuart Mill
When one's ideas are not challenged, one's ability to defend them weakens.
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 John Stuart Mill
One person with a belief is equal to ninety-nine who have only interests.
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 John Stuart Mill