“[Cf Cicero] He who knows only his own side knows little of that. His reasons may be good, and no one may be able to refute them. But if he is equally unable to refute the reasons on the opposite side: if he does not so much know what they are, he has no ground for preferring either opinion. The rational position for him would be suspension of judgment, and unless he contents himself with that, he is either led by authority, or adopts, like the generality of the world, the side which he feels most inclination. Nor is it enough that he should hear the arguments of adversaries from his own teachers, presented as they state them, and accompanied by what they offer as refutations. That is not the way to do justice to arguments, or bring them into real contact with his own mind. He must be able to hear them from people 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 the truth which meets and removes that difficulty. Ninety-nine in a hundred of what are called educated men are in this condition; even of those who can argue fluently for their opinions. Their conclusion may be true, but it might be false for anything they know; they have never thrown themselves in the mental position of those who think differently from them, and considered what such persons have to say; and consequently they do not, in any proper sense of the word, know the doctrine which they themselves profess. They do not know the parts of it which explain and justify the remainder; the considerations which show that a fact which seemingly conflicts with another is reconcilable with it, or that, of two apparently strong reasons, one and not the other ought to be preferred. All that part of the truth that turns the scale, and decides the judgment of a completely informed mind; they are strangers to; nor is it ever really known, but to those who have attended equally and impartially to both sides, and endeavoured to see the reasons of both in the strongest light. So essential is this discipline to a real understanding of moral and human subjects, that if opponents of all important truths do not do not exist, it is indispensable to imagine them, and supply them with the strongest arguments which the most skilful devils’ advocate can conjure up [bolding mine].”
John Stuart Mill. On Liberty. New York and Melbourne: The Walter Scott Publishing Company, Ltd. Chapter 2. pp 68-69
**Compare with Thomas Hobbes on controversy. A quote worth contemplating.