As a consulting parliamentary sketchwriter, I am often asked: "Is Tony Blair a liar?" People, some desperate, come to my chambers in SW1, unshaven, hollow–eyed, hair unkempt, pleading with me to answer. I give them a brief smile, pretend to be finishing off some notes with an expensive fountain pen, then fix them with an earnest gaze just above my glasses. After a brief but meaningful pause, I say: "Search me".

Alternatively, I could say: "It all depends on what you mean by 'liar', and how you define a 'lie'." He is clearly not a practitioner of the blatant, unequivocally mendacious whopper: Billy Bunter saying he didn't eat the cake, or a claim that you're working late at the office while you're bunked up with your secretary. Nor are we talking about the "no, it's delicious, it's just that I'm full", white lie.

Neither are we talking about lying of the two Jeffrey Archer types. Archer lied to save his skin over the Monica Coughlan affair. But he also fibs as a kind of hobby. Everyone who knows him has examples; one of my favourites involved a couple I know who met him at a party. As a gesture — he is indubitably a generous man — he offered to take them to dinner at the Ivy. Even you, they told him, can't get a table at the Ivy at no notice on a weeknight. But he smiled knowingly and disappeared briefly, returning to say they were expected.

At the end of the meal, while Jeffrey was in the Gents, the husband remarked to the maître d' that he had found a table for Lord Archer, but it would have been a different story for hoi polloi. "No, no," the maître d' told him, "Lord Archer booked this table three weeks ago."

So if Blair is a liar, it's not that kind of lie — though of course there were the rather surprising mentions of watching a player at Newcastle United who'd retired before Blair ever went there, and the mysterious claim to have stowed away on a plane. We can put those down to the kind of confusion children exhibit when they tell us about their imaginary friends. (In fact, politicians all have imaginary friends, or at least imaginary friendships. But they don't realise they're imaginary until it's too late.)

I guess I see our prime minister live, in action, roughly twice a week. What has always struck me is that he seems most slippery when telling what is, technically, the truth. As any student of the paranormal will tell you, deceivers and sleight–of–hand merchants need lashings of data from which to pick and choose. The briefly fashionable biorythms was a classic example: by generating hundreds of thousands of statistics for every human being on earth, it made it possible for the believer to prove anything. It works the same way in parapolitics. Targets and league tables produce a similar mass of figures which the politician can cherry–pick. If primary school English isn't looking good, head for secondary school maths. Poor GCSEs among children from low–income families? No matter: here's a two per cent improvement in science A levels in the north. The same applies to health statistics — more easily — since the system is now structured to produce more and better performance figures, without actually improving performance.

So when I see Blair reeling off figures at Question Time, I think he looks crafty and evasive, even though each fact, taken in isolation, is perfectly true. By contrast, in the run–up to the Iraq war, he appeared to be straight, direct and sincere. I remember early last year one session of the liaison committee when I thought: "This man really, truly believes what he is saying". At one point, asked why we seemed to be blindly following the Americans to war, he said, his voice husky with honesty, that if George Bush hadn't been planning to invade Iraq, he would have tried to persuade him. The speech in which he urged the Commons to approve the invasion was a masterpiece of convincing rhetoric, and one of the main reasons why so few Labour backbenchers rebelled.

So what went wrong? One thing the 'Bliar' brigade have to answer is this: if he was lying, what was in it for him? No politician can get to the top in Britain simply by sucking up to an American president. The enterprise was always very dangerous, militarily, financially, diplomatically and politically. Why did Blair risk losing all the goodwill he had built up in Europe, or the tens of millions the war cost, or the split in his own party?

My belief — or at least my assumption — is that he really did believe that Saddam was a serious threat to the world's stability and to our national safety. Misled by the intelligence (and by a Joint Intelligence Committee chairman, Sir John Scarlett, who seems to have been unwisely close to the political agenda) he genuinely felt the war was essential. It was an important part of his job to persuade us, which is why he presented himself to so many hostile audiences.

When you reach that position — and it's common with New Labour, and the whole Campbell/Mandelson approach to reality — mere facts exist either to illustrate a greater truth, or, if they fail to do that, to be ignored. The "could be deployed in 45 minutes" claim, which actually referred to low–range battlefield weapons rather than ballistic missiles, was either irrelevant, or else, presented in the right way, a convenient means of making the crucial case.

Is that a lie? Many people think so. The accumulation of evasions, massaged figures, uncomfortable facts discarded, all amount to lying, they feel. Tony Blair himself, I am almost certain, does not agree. Which is why he was so delighted by Hutton, and so distressed by the hoots of cynical derision with which his own exoneration was greeted.

So is he a liar? Not in his own judgement, not at all.

Simon Hoggart is parliamentary sketch writer for the Guardian