The test of real and vigorous thinking, the thinking which ascertains truths instead of dreaming dreams, is successful application to practice. - John Stuart Mill

That principle is, that the sole end for which mankind are warranted, individually or collectively in interfering with the liberty of action of any of their number, is self-protection. That the only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others.

When there are persons to be found, who form an exception to the apparent unanimity of the world on any subject, even if the world is in the right, it is always probable that dissentients have something worth hearing to say for themselves, and that truth would lose something by their silence.

The assumption that we are infallible can we justify the suppression of opinions we think false. Ages are as fallible as individuals, every age having held many opinions which subsequent ages have deemed not only false but absurd.