Mill argued in On Liberty (1859) that the whole of society could be swayed by the mere opinions and passions of a majority. For this reason, free speech was essential. Even if those who seek to suppress free speech are correct, if they are not willing to present their arguments afresh they might come to hold their correct conclusions as mere superstitions. Mill thus believed in freedom of opinion for its utility in promoting a generally rational pubic epistemology or shared theory of what constitutes knowledge.

He thought it was important that people have standards based on what is known as opposed to mere opinion. If there is free speech and public disagreement, then the parties that prevail have to give reasons for their views, according to Mill. In other words, Mill thought that free speech would encourage good arguments, and that good arguments would result in an informed public. Knowledge, according to Mill, required both reasoning and a justification of beliefs.