翻译的真让人难以理解啊，细读原文挺有意思，如下：

the claims of an opinion to be protected from public attack are rested not so much on its truth, as on its importance to society.

that none but bad men would desire to weaken these salutary beliefs; and there can be nothing wrong, it is thought, in restraining bad men, and prohibiting what only such men would wish to practise.

This mode of thinking makes the justification of restraints on discussion not a question of the truth of doctrines, but of their usefulness;and flatters itself by that means to escape the responsibility of claiming to be an infallible judge of opinions

Those who are on the side of received opinions, never fail to take all possible advantage of this plea; you do not find them handling the question of utility as if it could be completely abstracted from that of truth: on the contrary, it is, above all, because their doctrine is "the truth," that the knowledge or the belief of it is held to be so indispensable. There can be no fair discussion of the question of usefulness, when an argument so vital may be employed on one side, but not on the other. And in point of fact, when law or public feeling do not permit the truth of an opinion to be disputed, they are just as little tolerant of a denial of its usefulness.