It was one of the many curiosities of teaching graduate students in the US that whenever we turned to discussing Mill's essay On Liberty , they would spend all their time trying to imagine ways in which Mill's so-called harm principle could be extended to novel sorts of "harm". This was very odd. Mill's principle that coercion is to be exercised over anyone of adult years and tolerably sound mind only in order to prevent harm to others was very obviously meant by Mill to be read in conjunction with a narrow rather than an expansive view about what did and didn't harm us.

"I am shocked" is not the same as "I am harmed". "I am insulted" is not the same as "I am harmed". Anyone even halfway attracted by Mill's notion that people should only be forced to act in ways they do not want to for the sake of preventing harm to other people is supposed to be able to ask the question, "Am I harmed or do I just dislike it?"

This isn't a philosopher's question. Fifty years ago this year, the Wolfenden Committee's report on prostitution and homosexuality enunciated the view - acknowledging Mill - that the business of the law was "to preserve public order and decency, to protect the citizen from what is offensive and injurious" but otherwise to recognise that "there must remain a realm of private morality and immorality which is in brief and crude terms not the law's business".

The point of putting the issue like that was to get people to ask themselves the crucial question: when, if ever, and how, if ever, does the sexual conduct of someone else do me harm?

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For all the genuflection to Mill, it wasn't a question he asked. He put one sentence on decency into On Liberty and left it at that. It wasn't a question that Sir John Wolfenden's sponsors wanted asked; it took a decade to get Roy Jenkins's quite timid reforms. Even then Dick Crossman grumbled to his diary about wasting time on "buggers at Westminster" rather than the unemployed in Coventry.

At least the Jews of the Old Testament and the Emperor Justinian a millennium later had the right sort of argument - if not the facts - on their side. If God will visit you with fire and brimstone, it certainly does you harm if your neighbour engages in the wrong sort of sex, however privately he, she, or they may do it; and if Justinian had been right to think that buggery brought on earthquakes, you'd have been right to protest against its eventual decriminalisation by David Blunkett in 2001.

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Of course, the facts are otherwise. But unless you ask yourself the crucial questions, "Will it harm me and if so how?" the incentive to consider the facts is much diminished. You will be likely to think that what you don't like must be damaging. Wolfenden himself was not a great liberal by the standards of a later day. When he discovered that his son was gay, he wrote to ask him to stay away for a year or two; and he had no doubt that the corruption of the young was a serious issue. But he was both brave and honest.

Wolfenden was also more sensible than the experts who appeared before the committee. Almost all the psychiatrists who gave evidence claimed that homosexuality was a mental illness, a malfunction and a failure of development. Wolfenden took the rather more robust view that since homosexuals functioned as adequately as their straight colleagues in all walks of life, it was a pretty odd sort of illness whose only manifestation was the homosexuality itself.

So, what of my American graduate students? It's hard to know. It was almost always an attack from the Left. One version was to conflate argument with insult, so that anyone who might feel that an argument was going badly could claim to have been belittled; another was to invoke Foucault or Derrida according to taste and insist that it was all a matter of power relations or seizing the rhetorical high ground. It was in that sense all very academic.

And, reassuringly enough, it would never have occurred to any of them that there had once been a time when the Wolfenden report was a mould-breaking document.

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Alan Ryan is warden at New College, Oxford.