In fact, if anything the press has been too willing to give him the benefit of the doubt. Most reports I've seen are careful to use hedge words such as "alleged" or "reportedly," as if it were still a matter of conjecture whether he had "sexual relations" with the young White House intern. The president's denials are dutifully reported, as if there were some possibility he may be telling the truth.

But there isn't.

The minute Lewinsky signalled her willingness to testify they had an affair, there was no longer any room for doubt. He did it. She did it. They did it together. The president is lying. There is no other possibility.

This is not, I hasten to add, based on her credibility as a witness. On the contrary, we know she's a liar: the only question is whether she lied in describing their frequent sexual liaisons to former White House aide Linda Tripp on the now-famous secret tapes, or whether she lied in denying any such encounters in the affidavit she gave in the Paula Jones sexual harassment suit.

True, by all accounts, the tapes are pretty persuasive. It is hard to imagine a 24-year-old psychology major from Beverly Hills conjuring up the scene where Vernon Jordan tells her to deny there was any sexual affair, because "perjury is rarely prosecuted in civil cases." That still leaves us in the realm of speculation. So do all the other bits and pieces of circumstantial evidence of which we read -- the gifts, the phone calls, the visits, the witnesses -- however reliable or unreliable these reports may turn out to be. They might back her story. They might not.

But even liars are logical, if only in pursuit of their own self-interest. Follow their actions, follow the reasoning behind their actions, and the facts reveal themselves. It is not the tapes, ironically, but the affidavit, her sworn denial , that makes this an open and shut case. Or rather, it's her willingness to recant the affidavit. See if you agree.

There are only two possible scenarios: either she did it with the president, or she did not. Suppose she didn't do it. Let's say she's delusional, or a braggart, or whatever: anyway, she made up the whole thing. But somehow the story got out -- hence the subpoena from Jones's lawyers -- and now she's in big trouble.

Sometimes, caught in a lie, people are too scared to change their story, and dig themselves deeper. It's the most natural thing in the world. But she doesn't do that. Instead, she retracts the whole thing: swears out an affidavit saying that she never had sex with the president. Which is only fair, since under this scenario, that's the truth.

At which point, her troubles are over. She may have lied in what she thought was a private conversation with her "friend" Tripp. But she told the truth when it counted, in her affidavit. So as far as legal consequences go, she's home free. It's not perjury if you aren't under oath.

But if that is the case, then why all the lawyers? Why the lengthy negotiations with independent counsel Ken Starr? Why seek immunity from prosecution for a crime you didn't commit? And why agree to recant your previous sworn affidavit, just so you can perjure yourself by confessing to an affair that never took place? Obviously she's not doing any of this willingly: Starr is squeezing her, hard. But if she told the truth in her affidavit, what's his leverage?

So let's take the other scenario: that she did have sex with the president; that she told the truth in the tapes, but lied in her affidavit. In that event, her behaviour is wholly explicable. If she lied under oath, she's guilty of perjury.

And if it can be proved she lied under oath, she has every incentive to recant, in exchange for immunity. That she is willing to negotiate suggests Starr must have at least some corroborating evidence. But, when you think about it, her willingness to recant is all the proof you need. The only logical reason she would do so is if the affidavit was a lie.

That doesn't mean that every word she now says must be the truth. But if she was lying when she said they didn't have sex, then it must be true that they did have sex. Which, as far as Clinton is concerned, is more than embarrassing. It's the end.

Clinton, remember, didn't just lie on TV: he swore in his own deposition to the Jones case that he never had sex with Lewinsky. That's all you need. It doesn't matter whether he told her to lie, or Jordan told her to lie. It doesn't matter whether she's a tramp, or whether Starr is part of a right-wing conspiracy, or whether the media get things wrong, or anything else. If the president lied under oath, he's guilty of at least a crime, and possibly a misdemeanour.

Book your seats early for the impeachment hearings.