Pointing out that no fixed standard was recognized for testing the propriety of the interference on the part of the community with its individual members, he finds the test in self-protection, that is, the prevention of harm to others.  He bases the proposition not on abstract rights, but on “utility, in the largest sense, grounded on the permanent interests of man as a progressive being.”  He then uses the following argument to show that to silence opinion and discussion is always contrary to those permanent interests.  Those who would suppress an opinion (it is assumed that they are honest) deny its truth, but they are not infallible.  They may be wrong, or right, or partly wrong and partly right. (1) If they are wrong and the opinion they would crush is true, they have robbed, or done their utmost to rob, mankind of a truth.  They will say:  But we were justified, for we exercised our judgment to the best of our ability, and are we to be told that because our judgment is fallible we are not to use it?  We forbade the propagation of an opinion which we were sure was false and pernicious; this implies no greater claim to infallibility than any act done by public authority.  If we are to act at all, we must assume our own opinion to be true.  To this Mill acutely replies:  “There is the greatest difference

[238] between assuming an opinion to be true, because with every opportunity for contesting it it has not been refuted, and assuming its truth for the purpose of not permitting its refutation.  Complete liberty of contradicting and disproving our opinion is the very condition which justifies us in assuming its truth for purposes of action, and on no other terms can a being with human faculties have any rational assurance of being right.”

(2) If the received opinion which it is sought to protect against the intrusion of error is true, the suppression of discussion is still contrary to general utility.  A received opinion may happen to be true (it is very seldom entirely true); but a rational certainty that it is so can only be secured by the fact that it has been fully canvassed but has not been shaken.

Commoner and more important is (3) the case where the conflicting doctrines share the truth between them.  Here Mill has little difficulty in proving the utility of supplementing one-sided popular truths by other truths which popular opinion omits to consider.  And he observes that if either of the opinions which share the truth has a claim not merely to be tolerated but to be encouraged, it is the one which happens to be held by the minority, since this is the one “which

[239] for the time being represents the neglected interests.”  He takes the doctrines of Rousseau, which might conceivably have been suppressed as pernicious.  To the self-complacent eighteenth century those doctrines came as “a salutary shock, dislocating the compact mass of one-sided opinion.”  The current opinions were indeed nearer to the truth than Rousseau’s, they contained much less of error; “nevertheless there lay in Rousseau’s doctrine, and has floated down the stream of opinion along with it, a considerable amount of exactly those truths which the popular opinion wanted; and these are the deposit which we left behind when the flood subsided.”