Mill's background assumption was that ordinary morality was not so firmly founded or universally accepted a system, as to provide a sufficient condition for outlawing the behaviour of which it disapproved. There was need for another and more objective test, one that would enshrine and guarantee the fundamental liberal premise, which is that the law exists not to curtail our freedoms but to promote them. By preventing people from harming their neighbours, the law would promote the freedom of everyone.

Mill's argument has often been criticised, and this is not the place to examine the very many logical subtleties that have been devised both by its supporters and by its critics. Whatever its weaknesses, we must recognise that the argument has been and remains extremely influential, being pressed into the service of mid-19th-century liberal reforms until the present day. It captures one of the ground rules on which the public space of Western societies has been built - the rule that the law steps in only to defend fundamental freedoms and never to curtail them. If it seems to curtail them it is only in order to protect them from abuse. And freedom is abused whenever it is used by one person to remove freedom from another.

This was the argument deployed in the liberalising of the laws regarding sexual offences, and in particular those concerning homosexuality, in the 1960s. Some people, possibly a majority, thought homosexual acts to be immoral, whether or not between adults and whether or not consented to. But that was not a sufficient ground for making those acts illegal. Such was the argument advanced by the Wolfenden Report and embodied in the subsequent Sexual Offenders Act of 1967. It is significant, however, that those who adopted the argument and pressed for the reforms did not, on the whole, belong to the moral majority. Lord Wolfenden included the notorious amoralist and sex-obsessive Alfred Kinsey among his close advisers, and those who campaigned on the report's behalf tended to be members of the liberal elite for whom homosexuality had long been part of the culture. They did not, themselves, experience the clash between legal and moral norms that they were imposing on others and the reforms that they advocated were not, for them, a challenge that they had to swallow reluctantly, despite its bitter taste.