In On Liberty, first published in 1859, John Stuart Mill famously laid out four reasons for why free speech was a necessity in a free society: the first and second being that a contrary opinion may be true, or true in part, and therefore may require to be heard in order to correct your own erroneous views; the third and fourth being that even if the contrary opinion is in error, the airing of it may help to remind people of a truth and prevent its slippage into an ignorant dogma which may in time – if unchallenged – itself become lost.
