Again, trade is a social act. Whoever undertakes to sell any description of goods to the public, does what affects the interest of other persons, and of society in general; and thus his conduct, in principle, comes within the jurisdiction of society: accordingly, it was once held to be the duty of governments, in all cases which were considered of importance, to fix prices, and regulate the processes of manufacture. But it is now recognised, though not till after a long struggle, that both the cheapness and the good quality of commodities are most effectually provided for by leaving the producers and sellers perfectly free, under the sole check of equal freedom to the buyers for supplying themselves elsewhere. This is the so-called doctrine of Free Trade, which rests on grounds different from, though equally solid with, the principle of individual liberty asserted in this Essay. Restrictions on trade, or on production for purposes of trade, are indeed restraints; and all restraint, quâ restraint, is an evil: but the restraints in question affect only that part of conduct which society is competent to restrain, and are wrong solely because they do not really produce the results which it is desired to produce by them. As the principle of individual liberty is not involved in the doctrine of Free Trade, so neither is it in most of the questions which arise respecting the limits of that doctrine: as for example, what amount of public control is admissible for the prevention of fraud by adulteration; how far sanitary precautions, or arrangements to protect work-people employed in dangerous occupations, should be enforced on employers. Such questions involve considerations of liberty, only in so far as leaving people to themselves is always better, cæteris paribus, than controlling them: but that they may be legitimately controlled for these ends, is in principle undeniable. On the other hand, there are questions relating to interference with trade, which are essentially questions of liberty; such as the Maine Law, already touched upon; the prohibition of the importation of opium into China; the restriction of the sale of poisons; all cases, in short, where the object of the interference is to make it impossible or difficult to obtain a particular commodity. These interferences are objectionable, not as infringements on the liberty of the producer or seller, but on that of the buyer.
One of these examples, that of the sale of poisons, opens a new question; the proper limits of what may be called the functions of police; how far liberty may legitimately be invaded for the prevention of crime, or of accident. It is one of the undisputed functions of government to take precautions against crime before it has been committed, as well as to detect and punish it afterwards. The preventive function of government, however, is far more liable to be abused, to the prejudice of liberty, than the punitory function; for there is hardly any part of the legitimate freedom of action of a human being which would not admit of being represented, and fairly too, as increasing the facilities for some form or other of delinquency. Nevertheless, if a public authority, or even a private person, sees anyone evidently preparing to commit a crime, they are not bound to look on inactive until the crime is committed, but may interfere to prevent it. If poisons were never bought or used for any purpose except the commission of murder, it would be right to prohibit their manufacture and sale. They may, however, be wanted not only for innocent but for useful purposes, and restrictions cannot be imposed in the one case without operating in the other. Again, it is a proper office of public authority to guard against accidents. If either a public officer or anyone else saw a person attempting to cross a bridge which had been ascertained to be unsafe, and there were no time to warn him of his danger, they might seize him and turn him back, without any real infringement of his liberty; for liberty consists in doing what one desires, and he does not desire to fall into the river. Nevertheless, when there is not a certainty, but only a danger of mischief, no one but the person himself can judge of the sufficiency of the motive which may prompt him to incur the risk: in this case, therefore (unless he is a child, or delirious, or in some state of excitement or absorption incompatible with the full use of the reflecting faculty), he ought, I conceive, to be only warned of the danger; not forcibly prevented from exposing himself to it. Similar considerations, applied to such a question as the sale of poisons, may enable us to decide which among the possible modes of regulation are or are not contrary to principle. Such a precaution, for example, as that of labelling the drug with some word expressive of its dangerous character, may be enforced without violation of liberty: the buyer cannot wish not to know that the thing he possesses has poisonous qualities. But to require in all cases the certificate of a medical practitioner, would make it sometimes impossible, always expensive, to obtain the article for legitimate uses. The only mode apparent to me, in which difficulties may be thrown in the way of crime committed through this means, without any infringement, worth taking into
