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.