From John Stuart Mill's On Liberty, Chapter 2–“Of the Liberty of Thought and Discussion”: 

Our merely social intolerance kills no one, roots out no opinions, but induces men to disguise them, or to abstain from any active effort for their diffusion. With us, heretical opinions do not perceptibly gain, or even lose, ground in each decade or generation; they never blaze out far and wide, but continue to smoulder in the narrow circles of thinking and studious persons among whom they originate, without ever lighting up the general affairs of mankind with either a true or a deceptive light. And thus is kept up a state of things very satisfactory to some minds, because, without the unpleasant process of fining or imprisoning anybody, it maintains all prevailing opinions outwardly undisturbed, while it does not absolutely interdict the exercise of reason by dissentients afflicted with the malady of thought. A convenient plan for having peace in the intellectual world, and keeping all things going on therein very much as they do already. But the price paid for this sort of intellectual pacification, is the sacrifice of the entire moral courage of the human mind. A state of things in which a large portion of the most active and inquiring intellects find it advisable to keep the general principles and grounds of their convictions within their own breasts, and attempt, in what they address to the public, to fit as much as they can of their own conclusions to premises which they have internally renounced, cannot send forth the open, fearless characters, and logical, consistent intellects who once adorned the thinking world. The sort of men who can be looked for under it, are either mere conformers to commonplace, or time-servers for truth, whose arguments on all great subjects are meant for their hearers, and are not those which have convinced themselves. Those who avoid this alternative, do so by narrowing their thoughts and interest to things which can be spoken of without venturing within the region of principles, that is, to small practical matters, which would come right of themselves, if but the minds of mankind were strengthened and enlarged, and which will never be made effectually right until then: while that which would strengthen and enlarge men’s minds, free and daring speculation on the highest subjects, is abandoned.

Those in whose eyes this reticence on the part of heretics is no evil, should consider in the first place, that in consequence of it there is never any fair and thorough discussion of heretical opinions; and that such of them as could not stand such a discussion, though they may be prevented from spreading, do not disappear. But it is not the minds of heretics that are deteriorated most, by the ban placed on all inquiry which does not end in the orthodox conclusions. The greatest harm done is to those who are not heretics, and whose whole mental development is cramped, and their reason cowed, by the fear of heresy. Who can compute what the world loses in the multitude of promising intellects combined with timid characters, who dare not follow out any bold, vigorous, independent train of thought, lest it should land them in something which would admit of being considered irreligious or immoral? Among them we may occasionally see some man of deep conscientiousness, and subtle and refined understanding, who spends a life in sophisticating with an intellect which he cannot silence, and exhausts the resources of ingenuity in attempting to reconcile the promptings of his conscience and reason with orthodoxy, which yet he does not, perhaps, to the end succeed in doing. No one can be a great thinker who does not recognise, that as a thinker it is his first duty to follow his intellect to whatever conclusions it may lead. Truth gains more even by the errors of one who, with due study and preparation, thinks for himself, than by the true opinions of those who only hold them because they do not suffer themselves to think. Not that it is solely, or chiefly, to form great thinkers, that freedom of thinking is required. On the contrary, it is as much and even more indispensable, to enable average human beings to attain the mental stature which they are capable of. There have been, and may again be, great individual thinkers, in a general atmosphere of mental slavery. But there never has been, nor ever will be, in that atmosphere, an intellectually active people. When any people has made a temporary approach to such a character, it has been because the dread of heterodox speculation was for a time suspended. Where there is a tacit convention that principles are not to be disputed; where the discussion of the greatest questions which can occupy humanity is considered to be closed, we cannot hope to find that generally high scale of mental activity which has made some periods of history so remarkable. Never when controversy avoided the subjects which are large and important enough to kindle enthusiasm, was the mind of a people stirred up from its foundations, and the impulse given which raised even persons of the most ordinary intellect to something of the dignity of thinking beings.