Mill wanted truths to be tested and defended against argument so the truth was lived. In such circumstances the truth is alive and vivid. Then, and only then, truth can avoid being a dead truth.

That is why Mill says, “He who knows only his own side of the case knows little of that.”

To me it is like learning how to use a computer. It is not enough to be told how to use it. We have to use it to learn how to use it. Then the truth of how to use a computer becomes real.

So it is with reason.  A person might have been taught the reasons for an opinion, and those reasons might even be good reasons, but that is not good enough. If a person does not know what the reasons in favour of the opposite proposition are he really has no grounds to prefer either opinion. As Mill says,

“if he is equally unable to refute the reasons on the opposite side; if he does not so much as know what they are, he has no ground for preferring either opinion. The rational position for him would be to suspend judgment and unless he contents himself with that, he is either led by authority, or adopts, like the generality of the world, the side to 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 the arguments or bring them into real contact with his own mind.”

Neither authority nor desire is good grounds for a belief.  The only thing that works is vigorous open debate on both sides of a question with both sides able to argue their case fully and freely. We must experience fully the weight of the belief on the other side. We must know them in their most plausible and persuasive form. Nothing else will do. “He must feel the whole force of 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.” Unless one fully throws oneself into the position of the other we can never truly know what we profess to believe. We must see the arguments on both sides in the strongest light.

As a result of such reasoning, Mill makes a surprising and profound argument. He says, “So essential is this discipline to a real understanding of morals and human subjects, that if opponents of all important truths do not exist, it is indispensable to imagine them, and supply them with the strongest arguments, which the most skillful devil’s advocate can conjure up.”

Mill makes another suggestion.  He says, “mankind ought to have a rational assurance that all objections have been satisfactorily answered; and how are they to be answered if that which requires to be answered is not spoken? Or how can the answer be known to be satisfactory, if the objectors have no opportunity of showing that it is unsatisfactory.”

So free discussion is essential to understanding fully the opinion held. Its absence is harmful to the worth of the opinion. It is not enough that we hold true opinions, the process by which we gained those opinions is of critical importance.  Mill put it this way,

The fact, however, is, that not only the grounds of the opinion are forgotten in the absence of discussion, but too often the meaning of the opinion itself. The words which convey it cease to suggest ideas, or suggest only a small portion of those they were originally employed to communicate.  Instead of a vivid conception and a living belief, there remains only a few phrases retained by rote, or if any part, the shell and husk only of the meaning retained, the finer essence being lost.

If you want a vivid belief, and which indoctrinator does not want that, free discussion is an absolute prerequisite. Without it there is but a husk of a belief—again—a paltry thing. That is why absolute free discussion is so vital. Free speech brings life.

Like Joni Mitchell sort of said, “you must look at truth from both sides now.” Otherwise, it’s only truth’s illusions you will recall.