"There's no such thing as bad publicity." The phrase is often attributed to the American showman, Phineas T Barnum, which seems to have become the first line of defence in the media whenever a magazine or newspaper publishes candid and intrusive images of the rich and famous.
With telephoto lenses and modern mobile phones, there's no hiding place any more, whether you are a celebrity enjoying a discreet candlelit dinner with a glamorous assistant, or merely a politician stifling a yawn in the middle of a colleague's policy statement. However trifling your indiscretion, you can be sure it will be captured and transmitted for the delectation of the peoples of the world.
Yet if two individuals on the planet could be said to have a mature arrangement with the world's press, it would surely be the Duke and Duchess of Cambridge, Prince William and his wife, Kate. Arguably the world's most glamorous couple, they've avoided the worst excesses of the press by employing a skilful policy of mutually agreed guidelines, combined with a degree of cooperation with the insatiable demands of photographers, in exchange for the guarantee of a little privacy on the rare occasions when they're "off duty".
Yet this informal understanding with the media has been blown apart this week, after a Paris-based magazine published illicit photographs of the Duchess in a state of undress, taken by an opportunistic snapper during a private trip to a château in the French countryside.
While the magazine's editor, Laurence Pieau, defended the decision with a series of glutinous platitudes about wishing to show a "young couple in love" (before making veiled references to other, more explicit images still not published), the Royal couple has reacted with fury. "Incandescent" was the term used by one aide to describe Prince William's mood.
Yet perhaps the only surprise is that such a blatant violation hasn't occurred sooner. The couple's fragile understanding with the press was one based on discretion and goodwill, two commodities in short supply at the sleazier end of the publishing industry. Indeed, its frailty was amply demonstrated last month when candid images were published in the UK of Prince Harry at a party in Las Vegas.
The miscreant who took those particular snaps may have been an opportunistic partygoer rather than a gnarled paparazzi hack, but the newspapers were happy to publish them anyway.
While the current furore may be a storm in a teacup, it's impossible to ignore a resonance between Prince William's wife and the fate of his mother, Princess Diana. In one of the most prophetic statements ever made by a politician, the former Conservative minister Alan Clark wrote "one of these days someone is going to get killed" after watching Princess Diana's limousine furiously accelerating away from a typically chaotic press conference.
Alas, Mr Clark's speculation came calamitously true in 1997 when Princess Diana's car hit the stanchion of a gloomy underpass in the centre of Paris while fleeing photographers. Just how far some of the press will go was witnessed that night as photographers took snaps of the dying princess as she lay trapped in the car. No wonder Prince William views this fresh intrusion into his wife's privacy with such dread.
There are some who argue that the Duchess's decision to appear in a relative state of undress shows a naivete and that she was thus fair game. But what constitutes justifiable intrusion? Is it also permissible to photograph them inside their apartment through an open window? And if so, where do we draw the line? With them brushing their teeth? Having a bath?
Curiously there may be a silver lining. Now that the photographs are available, such images will surely lose some of their prurient attraction. Nudity, like swearing, soon loses its potency once it becomes commonplace. It may be cold comfort to the royal couple just now, but perhaps the public will get bored and leave them in peace.
And who knows? Perhaps the couple might feel it's a price worth paying. Even if they'll only admit it behind closed doors.
Michael Simkins is an actor and writer based in London
On Twitter: @michael_simkins