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 account, upon the liberty of those who desire the poisonous substance for other purposes, consists in providing what, in the apt language of Bentham, is called “preappointed evidence.” This provision is familiar to everyone in the case of contracts. It is usual and right that the law, when a contract is entered into, should require as the condition of its enforcing performance, that certain formalities should be observed, such as signatures, attestation of witnesses, and the like, in order that in case of subsequent dispute, there