" Mill believed the pursuit of truth required the collation and combination of ideas and propositions, even those that seem to be in opposition to each other. He urged us to allow others to speak-and then to listen to them- for three main reasons:

First, the other person's idea, however controversial it seems today, might turn out to be right. ("The opinion may possibly be true.")

Second, even if our opinion is largely correct, we hold it more rationally and securely as a result of being challenged. ("He who knows only his own side of the case, knows little of that.")

Third, and 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.") "

― John Stuart Mill , All Minus One: John Stuart Mill's Ideas on Free Speech Illustrated