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. The utmost they allow is an extenuation of its absolute necessity, or of the positive guilt of rejecting it. In order more fully to illustrate the mischief of denying a hearing to opinions because we, in our own judgment, have condemned them, it will be desirable to fix down the discussion to a concrete case; and I choose, by preference, the cases which are least favourable to me--in which the argument against freedom of opinion, both on the score of truth and on that of utility, is considered the strongest. Let the opinions impugned be the belief in a God and in a future state, or any of the commonly received doctrines of morality. To fight the battle on such ground, gives a great advantage to an unfair antagonist; since he will be sure to say (and many who have no desire to be unfair will say it internally), Are these the doctrines which you do not deem sufficiently certain to be taken under the protection of law? Is the belief in a God one of the opinions, to feel sure of which, you hold to be assuming infallibility? But I must be permitted to observe, that it is not the feeling sure of a doctrine (be it what it may) which I call an assumption of infallibility. It is the undertaking to decide that question _for others_, without allowing them to hear what can be said on the contrary side. And I denounce and reprobate this pretension not the less, if put forth on the side of my most solemn convictions. However positive any one's persuasion may be, not only of the falsity, but of the pernicious consequences--not only of the pernicious consequences, but (to adopt expressions which I altogether condemn) the immorality and impiety of an opinion; yet if, in pursuance of that private judgment, though backed by the public judgment of his country or his contemporaries, he prevents the opinion from being heard in its defence, he assumes infallibility. And so far from the assumption being less objectionable or less dangerous because the opinion is called immoral or impious, this is the case of all others in which it is most fatal. These are exactly the occasions on which the men of one generation commit those dreadful mistakes, which excite the astonishment and horror of posterity. It is among such that we find the instances memorable in history, when the arm of the law has been employed to root out the best men and the noblest doctrines; with deplorable success as to the men, though some of the doctrines have survived to be (as if in mockery) invoked, in defence of similar conduct towards those who dissent from _them_, or from their received interpretation.