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John Stuart Mill couldn’t do hypertext, but I can do it for him. John gave the second chapter of On Liberty the title "Of the Liberty of Thought and Discussion.“ Here is how he sums up his argument at the end of the chapter, with links to the posts where I give his more detailed arguments:

We have now recognised the necessity to the mental well-being of mankind (on which all their other well-being depends) of freedom of opinion, and freedom of the expression of opinion, on four distinct grounds; which we will now briefly recapitulate.First, if any opinion is compelled to silence, that opinion may, for aught we can certainly know, be true. To deny this is to assume our own infallibility. (See ”John Stuart Mill on the Adversary System,“ ”John Stuart Mill on the Protection of ‘Noble Lies’ from Criticism“ and ”Should Troubling Arguments Be Kept Away from Those Who Might Be Unduly Swayed by Them?“)Secondly, though the silenced opinion be an error, it may, and very commonly does, contain a portion of truth; and since the general or prevailing opinion on any subject is rarely or never the whole truth, it is only by the collision of adverse opinions that the remainder of the truth has any chance of being supplied. (See ”A Remedy for the One-Sidedness of the Human Mind“ and ”Why Progressives and Conservatives Need Each Other.“)Thirdly, even if the received opinion be not only true, but the whole truth; unless it is suffered to be, and actually is, vigorously and earnestly contested, it will, by most of those who receive it, be held in the manner of a prejudice, with little comprehension or feeling of its rational grounds. (See ”Let the Wrong Come to Me, For They Will Make Me More Right“ and "In Praise of Trolls.”)And not only this, but, fourthly, the meaning of the doctrine itself will be in danger of being lost, or enfeebled, and deprived of its vital effect on the character and conduct: the dogma becoming a mere formal profession, inefficacious for good, but cumbering the ground, and preventing the growth of any real and heartfelt conviction, from reason or personal experience. (See “How Freedom of Speech for Falsehood Keeps the Truth Alive.”)

Note: "John Stuart Mill’s Defense of Freedom" links to posts on the rest of "On Liberty."

Update, October 19, 2019: Richard Reeves and Jonathan Haidt have created an abridged version of this second chapter of On Liberty to make it’s still timely message more accessible to college students:

Update, October 31, 2019: You might be interested in John Danaher’s layout of John Stuart Mill’s argument for freedom of speech.