There is a convention in the British Army that field marshals never retire; they go on to half pay but remain on call until they die. That seems like a good rule for ex-prime ministers, to whom the public purse is more generous than it is to old soldiers. Yet there is this strange political convention dating back to 1990 that anyone who has ever been the tenant of 10 Downing Street must keep rigidly silent about domestic affairs.
That alone gives a rarity value to Tony Blair's cautious contribution yesterday to the public debate that has followed the August riots. In the past, he has been loquacious on the Middle East, which – given his record in office – is arguably the subject on which he is least qualified to give an opinion. He has also spoken about other interests, including Africa and religion, but never anything as close to home as the roots of civil unrest.
What is also striking about the essay he contributed to yesterday's Observer is how much more sensible he sounds, now that he is not asking us to vote for him. During most of those years when he ran the Labour Party he was the bland voice of received opinion, proclaiming the need to be bold, whilst he drew his wisdom from focus groups.
Yesterday, he made a series of claims that he would never have uttered if he was still looking ahead to the next election. British society, he wrote, is not broken; most young people have a better sense of social responsibility than their elders had 40 years ago, corporate greed is nothing new and may be lass rampant now than it used to be, and today's MPs are not idle shysters but do a job that is more demanding than it was a generation ago.
His most startling observation, though, concerned the famous speech he made as shadow Home Secretary in 1993, in the wake of the murder of two-year-old Jamie Bulger. At the time, it was so highly acclaimed that it marked the point at which Blair replaced Brown as the Labour Party's leader-in-waiting. Now he says that it was "good politics but bad policy". That is a distinction that serving prime ministers seldom grasp.
In another age, the idea that a former prime minister should simply shut up would have been considered absurd. Gladstone was a defeated former premier in 1874. He finally gave up 20 years later. Edward Heath was a brooding presence on the Tory benches for 26 years after his defeat in 1974. He, of course, is held up as a terrible example of how undignified it can be for a former leader to linger on, making trouble for his successor. But another way of looking at Heath's later years is that the old brute carried on talking sense while Thatcherite madness took over the Conservative Party.
It was his example that motivated Mrs Thatcher to retire to the House of Lords at the first opportunity. A controversy is now brewing about a new film in which the ageing Baroness, played by Meryl Streep, is portrayed as having lost contact with reality and being under the impression that her husband, Denis, is still alive.
That is fiction, and perhaps a little cruel, but the crueller fact is that the onset of old age has not been easy on the Baroness, who missed her 85th birthday party in Downing Street because she was not up to it. Considering that she spent a busy lifetime totally immersed in public affairs until the age of 65, when she suddenly felt compelled to go silent for life, it is not surprising if she found the passing years a mental strain.
The well-being of former prime ministers is their concern. What bothers me is the quality of political discourse. You do not need to be a member of the little Tony Blair fan club to think that this man, who ran the government for 10 years, must know something. Why he should have to keep it all to himself beats me.