Mill’s main concern was not government censorship. It was the stultifying consequences of social conformity, of a culture where deviation from a prescribed set of opinions is punished through peer pressure and the fear of ostracism.
In Mill’s view most likely, opposing views may each contain a portion of the truth, which need to be combined. (“Conflicting doctrines share the truth between them.”)
For free speech to be valuable to the pursuit of truth, we all need to be both humble and open.
We need humility to recognize that we might not be right about everything all of the time, and that we have something to learn from others.
All silencing of discussion is an assumption of infallibility.
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.
There is a class of persons (happily not quite so numerous as formerly) who think it enough if a person assents undoubtingly to what they think true, though he has no knowledge whatever of the grounds of the opinion, and could not make a tenable defence of it against the most superficial objections.
All languages and literatures are full of general observations on life, both as to what it is, and how to conduct oneself in it; observations which everybody knows, which everybody repeats, or hears with acquiescence, which are received as truisms, yet of which most people first truly learn the meaning, when experience, generally of a painful kind, has made it a reality to them.