John Rentoul: No, Tony has not lost his marbles. He just knows that he must not say the unsayable

There is method in the madness of Tony Blair. He cannot say sorry for the invasion of Iraq, because he does not accept that he made the wrong decision. This is a reasonable view, although many readers of this newspaper may disagree with it, but the Prime Minister sometimes presents it in such an unreasonable way that his critics resort to the language of psychology. He is delusional, they say, or, in the vogue phrase that Sir Malcolm Rifkind threw at him last week, he is "in denial".

Blair provoked his critics in the Commons on Wednesday by refusing to accept any responsibility for what he called the "wretched and inexcusable bloodshed" in Iraq. "The terrorists cause the terrorism," he told Sir Malcolm: a statement of grating moral simplicity in the style of the later Margaret Thatcher. Clever people who should have known better used to question her sanity too.

There are reasons for Blair's refusal to accept the obvious and, far from suggesting that he has forgotten where he put his marbles, they are testimony to the fact that he has his wits about him. It is precisely because he has a coolly sane grasp of the forces arrayed against him that John Humphrys was reduced to telling him he was "naive" on the Today programme. Humphrys tried and failed to get him to accept that Iraq would not be in the state it is if the Americans and British had not invaded. Andrew Marr had more luck in another part of the BBC last week by asking whether he felt "at some level culpable" for the present state of Iraq.

"It's not a question of being culpable," Blair replied. "Of course, I feel a deep sense of responsibility for putting the situation right. But you're putting it to me as if the reason why there is this problem in Iraq is because of British and American soldiers - it's not."

It takes the lay viewer or listener a little while to compute the calculations that Blair has thought through and that he carries out instantly. These calculations are lexicographical (culpable: "deserving blame") as well as tactical. There is a reason he will not accept the obvious fact that Iraq's present state is a consequence of the invasion four years ago. While it may be inescapable that he and George Bush bear some indirect responsibility for much that has happened in Iraq since March 2003, he can see where accepting such a concept would lead.

Would the front pages carry essays on contingent moral liability? Would there be a discussion on the Today programme about the extent to which the consequences of the invasion were foreseeable? No, he would be torn apart by the one-sided media that do not allow for nuance. He would be held personally accountable for every violent death in Iraq since the invasion. The headlines would read: "Blair - I have blood on my hands." Or similar.

He knows how the media work. That is why New Labour gained such a reputation for spin, and it is a reputation that is hung round his neck like a burning tyre now. He said in the Commons last week: "My experience over the past few years is that I am singularly incapable of spinning the media one way or another, particularly on this issue, on which it is incredibly difficult to get any balanced coverage at all."

Cue hollow laughter from all those who take it for granted that the only reason the House of Commons voted to join the US invasion was because of Blair's all-powerful spin. Yet balance would require that the concept of indirect responsibility is applied to both sides. If Blair is to take his share of the blame for the disastrous state of Iraq, then the invasion's opponents should accept that they wish Saddam Hussein were still in power.

That is not going to happen either, so parliamentary debates such as last week's are going to remain artificial, in which truth can be approached only by degrees. One Cabinet minister said to me that there had been a conscious attempt to "close the gap between rhetoric and reality" in recent months. That was why Blair agreed with David Frost in November, when he said Iraq had been "pretty much of a disaster". It was why Blair took David Cameron by surprise before Christmas by saying that "of course" he agreed with Robert Gates, the new US Defense Secretary, that "we" were not winning the war in Iraq. It was why he used the "wretched" phrase last week.

Instead of contributing to a balanced assessment, each step is taken as a cue for a new round of demands for an apology. I get the sense that Blair would like to say more. He seems almost as frustrated as his interrogators by the constraints imposed by the media culture, and often hints that he will have plenty to say when he leaves office. Last weekend he told Andrew Marr, "I'm happy to go". You do not need inside information - although I do have it - to know that this is less than the whole truth. He resents Labour MPs, and specifically Gordon Brown, who forced him out early.

The Labour Party's ambivalence towards him is encapsulated with succinct opacity by John Prescott, in an interview in the second of Michael Cockerell's films on the Blair years to be shown on BBC2 on Tuesday. Speaking of Iraq, and Blair's need, like the captain of a ship, to avoid showing the crew that he is concerned, he says: "Thank God I didn't have to take the decision, but I'm party to it so I don't walk away from it."

Blair simultaneously revels in his self-image as the sole decision-maker and resents the cowardice of his colleagues in rallying so cautiously to his bold lead. He thinks that he was forced out because Labour MPs who were glad he was there to take decisions they did not want to take, but were happy to follow them while they won elections, melted away the moment he had won the third.

So, no, he does not want to go. But there is a part of him that cannot wait to switch from defensive-politician mode to that of elder statesman seeking to nudge the elbows of historians as they write.

Another Cabinet minister said to me that the nub issue took the form of a history exam question of the future. "Could the Iraq occupation have been successful if different policies had been pursued?"

That is not an easy question to answer, yet it is a question that ministers can ask only in private while Blair is still Prime Minister - a full answer must await his departure. Until then, his refusal to accept the obvious is evidence not that he is in denial, but that he still has his head screwed on.

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