Number 10 cannot have known that Dr Kelly would commit suicide

After the first few days, it is impossible to tell where the Hutton Inquiry is going and equally impossible to doubt that it will get there. There is an aesthetic pleasure in the way that first-rate lawyers operate. James Dingemans' cross-examinations are forensic choreography. He clarifies the issues and cuts to the chase without browbeating his witnesses - although one suspects that if he needed to, he could and would. But there is something in his manner which makes it clear that economising with the truth is not an option. He and Lord Hutton have the inquiry in a tight intellectual grip.

Although we should know a lot more by the end of the week, after the Prime Minister's chief of staff Jonathan Powell and his foreign policy adviser David Manning have given evidence, it is now possible to have a stab at answering one of the biggest questions: why did Andrew Gilligan's sexing-up allegation arouse such anger in Downing Street, thus setting in train the events that precipitated Dr Kelly's suicide? There are three explanations, two of which are to the Government's credit. The third is not.

There is clearly a tacit convention on the supply of intelligence to ministers and the use which they make of it. The data itself is controlled by the professionals who are orchestrated by the Joint Intelligence Committee. The reports which the JIC produces are almost invariably nuanced, cautious and closely argued. Thereafter, a separation of powers comes into effect. The JIC has provided the evidence; ministers then decide how to use it. As long as the sources themselves were not compromised, the JIC would be inclined to let the politicians get on with it.

But Mr Gilligan's report appeared to cast doubt on the integrity of the JIC process and to imply that Richard Dearlove, the head of MI6, and John Scarlett, the Chairman of the JIC, had allowed Alastair Campbell to interfere with their material. Although no one who knows either man would believe that for a second, the Prime Minister felt compelled to defend the integrity of the intelligence services. That was commendable.

There is a further factor which Tony Blair would not be in a hurry to acknowledge, least of all in the Labour Party. The relationship between the PM and his foreign policy, intelligence and defence advisors is now as warm as it has ever been. Mr Blair; Richard Dearlove; John Scarlett; David Manning, who is now off to Washington as Ambassador; Michael Jay, the Permanent Secretary of the Foreign Office; Jonathan Powell, who came from the FO - they not only work together well on a basis of mutual respect: they like each other.

Matters were also helped by Robin Cook's departure from the Foreign Office in 2001. He had been a disruptive influence and Jack Straw is much easier. Like Geoff Hoon at Defence, Mr Straw accepts that the major decisions will be made in No 10.

This system has evolved to cater for the needs of a premier who has become increasingly involved with foreign affairs; increasingly at ease with both the subject matter and the personalities. There are parallels with Margaret Thatcher, but Mr Blair's machine runs more smoothly. He has a College of Cardinals, while Mrs Thatcher depended on Charles Powell alone.

These days, there is no conflict among the officials, while Anglo-American relations are as good as they were at the height of the Reagan-Thatcher partnership. During the build up and the Iraq war itself, David Manning was constantly on the phone to the President's National Security Advisor, Condoleezza Rice. They became firm friends. Equally, Richard Dearlove was ensuring that MI6 and the Americans would always work as colleagues and never as rivals. Again, Tony Blair would prefer it if the extensive and affectionate nature of the intelligence partnership was concealed from the Labour Party.

When people put in long and harmonious hours in each other's company while managing great events, a family atmosphere is created. That has happened in Number 10. After the intelligence members of the family appeared to be under attack from the BBC, the rest of the clan mobilised on their behalf. There is nothing discreditable about that.

Then the picture changes. With Mr Campbell's encouragement, Mr Blair was guilty of sexing up the presentation of the Iraqi threat, and thus of misleading the British people as to the reasons for going to war with Iraq. The PM may have convinced himself that there was an imminent threat from Iraq's weapons of mass destruction. Mr Blair has never found it hard to believe what was convenient to believe. At best, however, that was a breach of his responsibilities - and if he deliberately misled the people about the reasons for war, he was guilty of as grave a breach of trust as a Prime Minister can commit.

We may never know exactly what went on inside Mr Blair's head. But the intensity with which Number 10 reacted to the initial Gilligan report appeared to go beyond self-defence. It seemed to indicate guilt. This does not mean that Number 10 were guilty of killing Dr Kelly. It was not unreasonable of them to try to establish exactly what Dr Kelly had said, and to whom. It is also clear that Downing Street's real animus was directed against Andrew Gilligan and not David Kelly. They were hoping that Dr Kelly's testimony would discredit Mr Gilligan.

This is all rough stuff, but that does not make it illegitimate. After the Gilligan report, Dr Kelly must have expected to come under scrutiny, and there is no suggestion that he was unduly harassed. Nor was there anything in his past career to suggest that he would crack under the strain.

Richard Hatfield, the Personnel Director at the Ministry of Defence, denied that Dr Kelly was a senior civil servant. Mr Hatfield was correct, but only in a technical sense. Dr Kelly had received the CMG, a decoration only one below a knighthood, the level of award which would normally be given to a second-tier ambassador or to a highly-regarded civil servant retiring at Richard Hatfield's rank. Dr Kelly may not have been a senior civil servant, but he was a senior figure who should have been able to cope with rigorous questioning.

So what went wrong? Although this is speculative, the answer may lie in the onset of a previously unsuspected but crippling depression. Severe depression is a terrible illness, as anyone who has known a sufferer will testify. It is as if the victim had no outer layers of skin. Every aspect of existence is bruising and painful. The result can be a personality collapse, and suicide.

If that is what did happen to Dr Kelly, there are no grounds for blaming anyone else. Number 10 was not aware that they were dealing with a man in the depths of depression. It is probable that nobody else did either, until it was too late.

There seems no other rational explanation for Dr Kelly's death. If he did die in the throes of dreadful depression, that is a matter for profound sympathy. But it does not mean that he was playing the moral equivalent of the Ace of Trumps.