A witness takes the stand and gives testimony that may send someone to prison or ruin a company. How do we decide whether to believe them?
What sort of person was Dr David Kelly? What sort of witness was he when he gave oral evidence before the Commons foreign affairs committee about the government's dossier of 24 September 2002? What sort of person are you? What sort of witness are you when you give oral evidence to a court or arbitrator or adjudicator? And, if you were adjudicating the value of Dr Kelly's evidence, how would you decide?

And one more question: Would you, from what you saw or read of Dr Kelly's evidence, believe him? If you say yes, you're in good company. The parliamentary committee believed him. They gushed about him. They believed he was not journalist Andrew Gilligan's primary source of information. But if I had been adjudicating the value of his evidence, I would have been pretty darn close to rejecting it completely.

People who adjudicate – by that I mean judges, magistrates, jurors, arbitrators, adjudicators and tribunal members of all kinds – don't get much training about "sorts" of people. The idea is to leave the adjudicator to their feel, intuition, sense about the witness. God knows, that is dangerous. Dr Kelly came over as a really nice sort, didn't he? It took less than 45 minutes to fathom that. I am not about to say he was not a nice guy. No, no. But what I do know is that some witnesses are very good at being a witness but in truth love to cheat, love to spar, love to lie, love to compete. And it's a hell of a job to crack these blighters, because they conduct their whole lives like that.

And if there are sorts who have no compunction about pulling a fast one on you, who delight in sleight of hand in their business affairs, and will happily con not just the other bloke but a lowly adjudicator, too; then there are those who make bad witnesses but who are absolutely straight. These straight witnesses are dangerous. The witness who stumbles with his words, changes his mind, gets confused, is inconsistent, shifts in his seat, looks like a guilty man in a police interrogation is a complete danger to himself, the tribunal, to the world, simply because he doesn't come over as straight. Have you got the picture?

And when I learned about the unfortunate Dr Kelly, I became frightened. Strong word, but I mean it. I am frightened about the process of deciding whether evidence is sound or not. Frightened because in the building dispute business, decisions are made on the basis of how a witness comes over. People in the crown court go to jail because of the perceived reliability of witnesses. We hear the evidence without knowing much about the witnesses themselves, or their ethics, culture, sort. Nor in a short cross-examination, is there much time to probe. And in building adjudications and some arbitrations the decision gets made after merely gawping at written submissions. I am frightened of being fooled; that's the sort I am.

Some witnesses are very good at being a witness but in truth love to cheat, fence, lie, compete. It takes hellish work to break these blighters

And why would I get close to rejecting Dr Kelly's evidence? Well, I have read the full transcript of the examination. It was, apart from one isolated piece of rudeness by an apologetic MP, a good inquiry. Its tone and depth was exemplary. Dr Kelly was moved along swiftly and deftly. He was given time to answer. He was not bullied or tricked or cut short. The committee welcomed his frank and open responses. But then I read Thomas Stuttaford's medical opinion in The Times and it startled me. He said: "There cannot have been a doctor in the country who, when watching David Kelly being interviewed by the select committee did not feel that they were privy to the sight of a seriously clinically depressed patient being tortured by the circumstances in which he had found himself."

He showed, said Stuttaford, the appearance of hopeless bewilderment; it was in his speech pattern, expression, gestures and posture. His shoulders were bowed, his eyes averted and his lacklustre look betrayed his misery. In which case, if Stuttaford is right, this witness was wholly unfit to give evidence. He was on the edge of suicide. No wonder he said he could not remember what had happened or a fact was not in his recollection.