Page:On Liberty (4th Edition).djvu/174

174 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 every one 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 may be evidence to prove that the contract was really entered into, and that there was nothing in the circumstances to render it legally invalid: the effect being, to throw great obstacles in the way of fictitious contracts, or contracts made in circumstances which, if known, would destroy their validity. Precautions of a similar nature might be enforced in the sale of articles adapted to be instruments of crime. The seller, for example, might be required to enter in a register the exact time of the transaction, the name and address of the buyer, the precise quality and quantity sold; to ask the purpose for which it was wanted, and record the answer he received. When there was no medical prescription, the presence of some third person might be required, to bring home the fact to the purchaser, in case there should afterwards be reason to believe that the article had