[13] concede that the expression of opinion is a right of the same kind, it is impossible to contend that on this ground it can claim immunity from interference or that society acts unjustly in regulating it. But the concession is too large. For whereas in the other cases the limitations affect the conduct of every one, restrictions on freedom of opinion affect only the comparatively small number who have any opinions, revolutionary or unconventional, to express. The truth is that no valid argument can be founded on the conception of natural rights, because it involves an untenable theory of the relations between society and its members. On the other hand, those who have the responsibility of governing a society can argue that it is as incumbent on them to prohibit the circulation of pernicious opinions as to prohibit any anti-social actions. They can argue that a man may do far more harm by propagating anti-social doctrines than by stealing his neighbour’s horse or making love to his neighbour’s wife. They are responsible for the welfare of the State, and if they are convinced that an opinion is dangerous, by menacing the political, religious, or moral assumptions on which the society is based, it is their duty to protect society against it, as against any other danger. [14] The true answer to this argument for limiting freedom of thought will appear in due course. It was far from obvious. A long time was needed to arrive at the conclusion that coercion of opinion is a mistake, and only a part of the world is yet convinced. That conclusion, so far as I can judge, is the most important ever reached by men. It was the issue of a continuous struggle between authority and reason—the subject of this volume. The word authority requires some comment. If you ask somebody how he knows something, he may say, “I have it on good authority,” or, “I read it in a book,” or, “It is a matter of common knowledge,” or, “I learned it at school.” Any of these replies means that he has accepted information from others, trusting in their knowledge, without verifying their statements or thinking the matter out for himself. And the greater part of most men’s knowledge and beliefs is of this kind, taken without verification from their parents, teachers, acquaintances, books, newspapers. When an English boy learns French, he takes the conjugations and the meanings of the words on the authority of his teacher or his grammar. The fact that in a certain place, marked on the map, there is a populous city called Calcutta, is for most [15] people a fact accepted on authority. So is the existence of Napoleon or Julius Caesar. Familiar astronomical facts are known only in the same way, except by those who have studied astronomy. It is obvious that every one’s knowledge would be very limited indeed, if we were not justified in accepting facts on the authority of others.