However, the biological fact that procreation normally requires a man and a woman surely indicates that society has a unique interest in preserving this particular form of commitment. And in fact all civilised societies have always recognised this fact in the ceremonies and legal institutions that together constitute marriage. The code of conduct on which Cameron relies, the code of the gentleman, is ultimately based on the medieval code of chivalry, which was primarily directed towards the "gentle" treatment of women by warriors. Without that give and take between masculine and feminine interests, without the gradual emergence of romantic love between equals, alongside children and property, as the sine qua non of marriage, the institution would never have become what it is, with its place at the very heart of humanity. Marriage, during a history almost as long as that of humanity itself, has flourished only when society came to recognise the unique contribution that a woman makes to each generation when she is cared for by a man during the vulnerable childbearing years. To abolish that uniqueness was the great injustice of the welfare state, which legitimised illegitimacy by making single parenthood the norm in many communities. Once marriage was no longer the prerequisite for procreation, men became redundant and women became dependent on the state. Now, when marriage is no longer the destiny (though it remains the hope) of a majority of society, a Conservative politician proposes not only to make men inessential for marriage but women too. "Marriage is not just a piece of paper," Cameron told the Tories in Manchester. No, indeed it is not; but by the time the Prime Minister has finished with it, marriage will be just that: a piece of paper signifying legal status and not much more. It will have lost its organic connection with posterity.

Cameron's difficulties with girls tell us something about the state of the nation. We have become more relaxed about moral questions in general: one only has to consider what public figures get away with in private life. I recently heard a leading Tory politician compare Boris Johnson, the Mayor of London, to Lord Palmerston, the Victorian Prime Minister of whom Disraeli supposedly said: "He is now 70. If he could prove evidence of his potency in his electoral address he'd sweep the country." Though polls suggest that Labour is 20 points ahead in London, Boris is quite likely to sweep the capital next year. He has become a fixture in British comedy, replacing his bitter rival Ken Livingstone, who has lost his "cheeky chappy" persona. Yet it is a mistake to assume that anything goes, or that people are less inclined to make harsh judgments about perceived moral failings in public life. David Laws and Liam Fox resigned for such reasons. As I write, the fate of one Cabinet minister (Chris Huhne) hangs in the balance. What determines their fates is, in part, the question of whether they can be trusted to behave as gentlemen, in the eyes of their peers and the public. 

How, though, do we judge whether politicians have fallen below acceptable standards of conduct when we no longer agree on what is reasonable to expect? Can reason provide objective criteria by which to judge political, or indeed any human conduct? Back in 1948, two great philosophers, Michael Oakeshott and Karl Popper, exchanged letters on this subject; they may be read in the online archives of the Hoover Institution. Oakeshott's big idea was the pernicious impact of rationalism in politics: "You see, I don't believe that reason is the only bond which unites men, not because men are unreasonable sometimes, but because there is something else much stronger that unites them, e.g. a common civilisation (where it exists), common habits of behaviours (where they exist) — neither of which are rational, dependent upon argument or common to all men. There is nothing, I think, common to all men." Popper agreed with Oakeshott's critique, as far as it went: "You have criticised ‘rationalism'. But have you tried to see the needs, desires, hopes, etc. to which it answers, in proper perspective?" Oakeshott is an industry in the US, but I doubt that many of the Cabinet have read him.