One of Mill’s most cherished beliefs was that there should be complete freedom of thought and discussion. He devotes almost a third of On Liberty to these vital freedoms, while accepting that there should sometimes be limits to what one is permitted to say in public.

The first thing to note, for Mill, is that the fact a view is unpopular is no reason at all to silence it: ‘If all mankind minus one were of one opinion, and only one person were of the contrary opinion, mankind would be no more justified in silencing that one person, than he, if he had the power, would be justified in silencing mankind’ (On Liberty, 142). In fact, Mill argues, we have very good reason to welcome the advocacy even of unpopular views. To suppress them would be to ‘rob the human race, posterity as well as the existing generation’. How so? Well, Mill argues that, whether the controversial view is true, false, or a mix of the two, we will never gain by refusing it a voice. If we suppress a true view (or one that is partially true) then we lose the chance to exchange error, whole or partial, for truth. But if we suppress a false view we lose in a different way: to challenge, reconsider, and perhaps reaffirm, our true views. So there is nothing to gain by suppression, whatever the truth of the view in question.

(Jonathan Wolff, An Introduction to Political Philosophy, rev. ed. [New York: Oxford University Press, 2006], 107)