It is not another nine-eleven - any more than nine-eleven was a nuclear strike.
It was not even another Madrid. Tony Blair was forced to step off the world stage at Gleneagles, but his Government is not in danger. If anything it is likely to come out of yesterday's events stronger. It is almost inconceivable that British troops will be removed hastily from Iraq, the way the Spaniards were last year.
Yesterday's attacks on London are a shocking reminder that terrorists who claim to be associated with al-Qaida are still in business, still murderous, still capable of inflicting vicious harm, and now targeting Britain. They have ended or ruined the lives of hundreds of innocent individual victims.
They have demolished smug suggestions that policemen were indulging in selfserving panic-talk when they insisted that it was a matter of "when, not whether" something like this would happen. They have also exposed one aspect of the folly of holding the Olympics in Britain - but we knew that.
Yet even amid yesterday's mayhem, you could not escape the thought that it could have been terribly worse. In the City, things went astonishingly right.
The Bank of England's monetary policy committee carried on with its deliberations and delivered its interest rate decision (albeit the wrong one) on the stroke of noon.
Banks and brokers whose offices were uncomfortably close to one or more of the bombs moved their activities to premises elsewhere. Backup plans seem to have worked.
The Stock Exchange sensibly asked traders to switch off computerised trading systems that could have caused havoc. But it stayed open. Remember, Wall Street was out of action altogether for more than a week after the attacks on the twin towers - and did heroically to re-open when it did.
The verdict of the market was reassuring, too. It began to have second thoughts about the initial markdown within a couple of hours. A 71-point loss on the Footsie by the close may turn out to be an understatement of the economic consequences of yesterday's events - particularly if there is more to come.
But lack of panic has a value in its own right on days like that. With just a little luck it could become self-fulfilling and help steady nerves throughout the economy.