Many years ago, when I was running the news desk of a national newspaper, I took a phone call one evening from a woman who declined to give her name but asked: "Are you interested in a story about the private life of a leading British politician?"
I told her that I might be, but only if the way the politician conducted himself in private was directly relevant to his public responsibilities. "Pity," she said, and hung up.
A few weeks later, the papers were full of stories about how a very senior politician had had an extra-marital affair. It caused a huge amount of excitement at the time, but the politician's opinion poll ratings went up, as did his party's, and he is still a respected public figure to this day. (There is no need, after all this time, to name him, although many of you will probably be able to guess whom I'm referring to.)
As a journalist, I was probably wrong to turn down the chance of a major scoop. If I had to make the same decision today, I would probably be less squeamish. But I was interested to see this week that, just days after we learned of president François Hollande's troubled private life, the European Court of Human Rights set out some useful guidelines about where it thinks the line should be drawn when it comes to the private lives of public figures.
The case concerned a book published in 2007 by a former girlfriend of the then Finnish prime minister, Matti Vanhanen. At the time of their affair, she was unmarried and he was divorced. The book's author, Susan Ruusunen, and her publisher were both prosecuted under Finland's privacy laws - they were first acquitted, but after the prosecution appealed, the verdict was overturned.
Finland's Supreme Court upheld the appeal court verdict, but it significantly narrowed the grounds on which it upheld their conviction. "The only references which, according to the court, had illegally disclosed information about the prime minister's private life were the information and hints about the sex life and intimate events between the girlfriend and the prime minister... descriptions of their brief and passionate intimate moments as well as giving massages to each other, and accounts of their sexual intercourse."
So, in Finland at least, it's a criminal offence to disclose intimate details of a prime minister's sex life. Which brings us to the key question: where exactly should we daw the line between protecting freedom of expression and protecting an individual's right to privacy? The European court's ruling, as you'd expect, is both lengthy and complex. But I've managed to plough through it for you - here, for example, is the all-important freedom of expression principle as set out by the court's judges (it's paragraphs 44 and 45, if you care to look them up for yourself):
"Although the press must not overstep certain bounds, particularly as regards the reputation and rights of others and the need to prevent the disclosure of confidential information, its duty is nevertheless to impart - in a manner consistent with its obligations and responsibilities - information and ideas on all matters of public interest... The safeguard afforded by Article 10 [of the European Convention on Human Rights] to journalists in relation to reporting on issues of general interest is subject to the proviso that they act in good faith in order to provide accurate and reliable information in accordance with the ethics of journalism. In addition, the court is mindful of the fact that journalistic freedom also covers possible recourse to a degree of exaggeration, or even provocation."
The Strasbourg court accepts that the right of politicians to have their privacy protected is less than that of lesser mortals: by virtue of holding public office, they are rightly subject to closer scrutiny of their honesty and judgment. ("The limits of permissible criticism are wider as regards a politician than as regards a private individual. Unlike the latter, the former inevitably and knowingly lay themselves open to close scrutiny of their words and deeds by journalists and the public at large, and they must consequently display a greater degree of tolerance." - Paragraph 46)
So the court's conclusion is that most of what the former Finnish prime minister's former girlfriend wrote about their affair was perfectly justifiable. "The information about how and when the former Prime Minister had met the girlfriend and how quickly their relationship had developed had relevance to general public discussion as these issues raised the question of whether in this respect he had been dishonest and lacked judgment. The [Finnish] Supreme Court also found that the information concerning the great differences in the standard of living between the girlfriend and the former prime minister, his lifestyle, the data protection concerns and the protection of the highest political authorities in general had relevance to general public discussion. The court agrees with this. From the point of view of the general public's right to receive information about matters of public interest, there were thus justified grounds for publishing the book."
All of this is of direct relevance to the French media as they tread warily through the tangled thickets of their president's love life. If my reading of the Strasbourg ruling is right, they're perfectly okay under European law to disclose his affair, because there is a justified public interest in how a political leader conducts himself in private. (And if there were any doubts, his deeply embarrassing inability to answer a question about who is France's current first lady should surely have dispelled them.)
But, according to the Strasbourg judges, what happens between the sheets should stay between the sheets. Once the bedroom door is closed, it stays closed - even for presidents and prime ministers.
Follow Robin Lustig on Twitter: www.twitter.com/@robinlustig