Is John Edwards’s admitted extramarital affair, previously bruited about the blogosphere and the tabloid press, his business or the public’s business? Can he sue anyone for invasion of privacy?

If an English High Court can award nearly $1 million to Max Mosley, tsar of grand prix motor racing, for invasion of privacy against Rupert Murdoch’s London tabloid, News of the World, which exposed Mr. Mosley’s participation in a sadomasochistic “orgy,” why can’t Mr. Edwards, a seasoned plaintiff’s lawyer, sue too? And why can’t Eliot Spitzer’s favorite hooker, Ashley Dupre, sue Mr. Murdoch’s New York Post for revealing her recent matinee fling in New York’s Gramercy Park Hotel?

At least, why can’t Ms. Dupre’s married companion, TJ Earle, sue Mr. Murdoch for outing his tryst with Ms. Dupre? And why can’t Barack Obama claim that the Israeli daily newspaper that published the purloined personal letter he left in the crack of Jerusalem’s Western Wall invaded his privacy as well? The reasons are a legal imbroglio.

Mr. Mosley, 68, son of England’s fascist leader of the 1930s, was caught on tape at a five-hour Chelsea party in London where, surrounded by five female prostitutes, he acted out a “prison scene.” The women, clad in striped costumes, were subjected to beatings, verbal abuse, and humiliation. Mr. Mosley was struck 21 times on his bottom by a dominatrix. The orgy took on a smattering of Nazi overtones. When Mr. Mosley spanked a half naked woman, he called out the count in a guttural German. One dominatrix wore a Luftwaffe uniform; another, to whom Mr. Mosley spoke in German, wore a military “guard’s” uniform; and a third protested, “But we are the Aryan race, the blondes.”

Mr. Murdoch’s company secretly taped Mr. Mosley’s escapade, after it paid one of the women $100,000 to help make the arrangements. Mr. Mosley promptly sued for invasion of privacy.

Denying any Nazi theme to the orgy, Mr. Mosley testified candidly of his 45 year old proclivity for sadomasochism, which he described as a “perfectly harmless activity provided it is between consenting adults who want to do it, are of sound mind, and it is in private.” One of the dominatrices testified that sadomasochism might not be “everybody’s cup of tea,” but that she would rather do it “by far than go to the dentist.” Mr. Mosley admitted he gave one of the five women $70,000 for their favors.

Although the English High Court judge held that Mr. Mosley was the victim of a press sting operation for which there was no public interest justification, he denied Mr. Mosley millions in punitive damages. The Court rested its judgment against Mr. Murdoch’s publication upon a provision of the European Convention on Human Rights to which England is signatory.

That Convention establishes a broad right of privacy; it also contains a provision protecting press freedoms, which, some argued, were unduly abridged by the judgment. The balance between the two is delicate. Commenting on the judgment, Mr. Mosley declared, “I feel very strongly that some newspapers literally ruin people’s lives and more has to be done to stop this.”

The Mosley, Dupre, Earle, and Obama cases present distinct fact patterns. Ms. Dupre was originally outed by a “source” at the Emperor’s Club who arranged the ill-starred date. Her connection with Mr. Spitzer undoubtedly extinguished her privacy rights because she was referred to in a federal criminal complaint. But how long does her status as a “quasi-public figure” continue? Is she to be forever branded with a scarlet letter?

The Post broke the story of her liaison with Mr. Earle five months after she had been with Mr. Spitzer and four months after Mr. Spitzer left office. Is there a point in time when Ms. Dupre, a private person involuntarily thrust into the vortex of public opinion, becomes legally entitled to a little privacy in her life? And what of Mr. Earle, who is not, and never was, a public figure?

Undoubtedly, few have ever heard of him outside of Wall Township, N.J., where he and his brothers have a road construction business. Does Mr. Earle give up his right of privacy merely because of his relationship with Ms. Dupre? One would think not.

And Mr. Obama, as public a figure as any could be, must certainly have some privacy right to write a letter praying for himself and his family, and to place the message in a crack in the wall of the temple of Solomon without fear that it will be stolen and sold to a press organization eager to publish it.

At one point, there was no common law right of privacy, and there may be none today in many states. In America, the call for such a right, sometimes described as the right “to be let alone,” originated in 1890 with a Harvard Law Review article by Samuel Warren and his law partner, later to become Supreme Court Justice Louis Brandeis. The common law has always recognized “a man’s house as his castle,” they concluded. “Shall the courts thus close the front entrance to constituted authority, and open wide the back door to idle or prurient curiosity?”

But, even Warren and Brandeis recognized that the right of privacy was not unbounded with the reasonableness of the publication in light of all the surrounding circumstances. “There are persons,” they wrote, “who may reasonably claim as a right, protection from the notoriety entailed by being made victims of journalistic enterprise. There are others who, in varying degrees, have renounced the right to live their lives screened from public observation.” Thus, in the authors’ view, the less public the figure, the greater the zone of privacy.

Our curiosity, at times prurient, about public figures and celebrities is virtually insatiable. The press knows that to feed it is to make money. Sex sells. Often, sensational revelations about private lives are in the public interest and help shape our attitudes toward public officials. After all, Brandeis said, “sunlight is the best disinfectant.”

Our First Amendment protects press freedoms to a far greater extent than the English law does. But even public figures have private lives. What is protected expression may still be an ethically questionable invasion of privacy.

Mr. Zirin, a New York trial lawyer, is a former assistant United States attorney. He is co-host of the cable television talk show “Digital Age,” where he has discussed privacy issues on the Internet.