By contrast moral rules or principles cannot be brought into being or changed or eliminated in this way. To assert that this ‘cannot’ be is not, however, to deny that some conceivable state of affairs is actually the case, as the assertion that human beings ‘cannot’ alter the climate would be. Instead this assertion points to the following facts. It is perfectly good sense to say such things as ‘As from 1 January 1960 it will be a criminal offence to do so-and-so’ or ‘As from 1 January 1960 it will be no longer illegal to do so-and-so’ and to support such statements by reference to laws which have been enacted or repealed. By contrast such statements as ‘As from tomorrow it will no longer be immoral to do so-and-so’ or ‘On 1 January last it became immoral to do so-and-so’ and attempts to support these by reference to deliberate enactment would be astonishing paradoxes, if not senseless.

H. L. A Hart, The Concept of Law, loc. 443