In commenting on President Obama’s address to the United Nations General Assembly and his defense of freedom of expression, Paul Pillar notes: [T]he president’s presentation overlooked the single most important reason to safeguard free speech: it is one of the best ways to get closer to the truth — and to what is effective and to what works. John Stuart Mill, in his essay On Liberty, identified this as the primary reason for safeguarding freedom of thought and discussion:

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.

Mill went on to explain further reasons for ensuring free expression and how the subject is not just a simple matter of something being true or false:

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.

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. 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.

An address by a head of government at the United Nations is not the same as a discourse by a political philosopher, and it probably would have been unwise to try to work this kind of reasoning into the president’s speech this week. But we should keep in mind this most fundamental set of reasons not only for why freedom of speech is something we cherish but also why it should be cherished, even when it appears to collide with, say, someone’s religious faith.

We should keep it in mind partly because the ills of restricted expression that Mill described sometimes infect our own public discourse, notwithstanding the constitutional guarantees of the First Amendment. This does not always take the form of false dogma imposing itself, although we see that in, for example, creationist attempts to affect school curricula. More often it involves the political correctness involved in automatic acceptance of a “general or prevailing opinion” — as it might relate to, for example, a foreign alliance or a perceived foreign threat — and quickness in shouting down those who challenge such an opinion.