Listen carefully, Bond. The rules have changed ...

New enemies and an information explosion have turned today's spies into thinkers, not shooters

In 1971, Edward Heath's government pulled off one of the most bracingly inspired diplomatic coups of the post-war period. Britain expelled 105 Soviet spies. This was definitely not cricket. "Tit for tat" expulsions were fine, but a pre-emptive mass eviction was unheard of. The Russians were, literally, speechless.

It was a brilliant move because some genius at the Foreign Office had realised that spying, in those days, was not a zero sum game. The Soviets had dozens, possibly hundreds, of agents moving about a relatively open country. To keep track of each one, with their dead letter boxes in surreally banal places like Crouch End, required nine MI5 operatives. Of course, we also had agents in the Soviet Union. Some were successful, but on the whole spying on a tightly controlled police state did not produce the same returns as spying on a democracy. We had, therefore, nothing to lose by refusing to play by the rules.

Things have changed. Russia is no longer an enemy, but, on the other hand, it must be an easier place to be a spy now that it is free of Communist control. And it is a necessary place on which to spy. It remains a potentially unstable nation with a vast nuclear arsenal. Knowing what is going on there is probably more important than knowing about countries - such as Libya and Iraq - that are clearly defined as threats. In addition, the world as a whole remains a dangerous and unpredictable place. The paranoid nation state may be resting, but it is not dead. The great game of covert diplomacy must go on.

Yet the news that the Russians are to expel British diplomats for spying comes as a shock, as if Slade or the Bay City Rollers suddenly had a number one hit. It is news from another era, from a time when James Bond would defuse the wicked hi-tech schemes of the KGB. Somehow, we thought, we had grown out of all that.

And it wasn't just the end of the Cold War that made spying seem dated. There has also been the steady stream of revelations from Oleg Gordievsky and others. These exposed not the exciting world of James Bond, but a drab world of clerks and bureaucrats, labouring as much against the demands of their bosses as against the enemy. Plus everybody did it and nobody could provide any objective assessment of gains or losses. It seemed, therefore, an essentially futile activity, a mere ritual whose silliness made it appear contemptible rather than heroic.

But this Russian capture - I am assuming, almost certainly correctly, that we were indeed spying - reminds us that the ritual must continue. It must continue even though it may seem emptier than ever. The primary cause has gone. Our spies are no longer protecting our freedom against a savage totalitarianism. Rather, they are protecting us against a whole range of more nebulous industrial and economic threats, and an uncertain military environment. Their activities cannot be justified by the formula us-good, them-bad; they can only be justified by the generalised conviction that it is better to know than not to know, better to be ready than to be taken by surprise.

Ultimately, however, traditional spying must be on the way out. Certainly, there is more to know than ever before, and as much need to know. But, at the same time, there is far more knowledge that is available to everybody. The spook rifling a filing cabinet or skulking around an air base is, increasingly, an unnecessarily risky use of manpower. Satellites and computers can penetrate frontiers far more effectively, and the averagely gifted teen hacker can find his way into all kinds of secret systems. Information is everywhere, lying about the place like rubbish on a skip; and, also like rubbish on a skip, some of it can turn out to be surprisingly valuable.

What now counts is understanding. We know, more or less exactly, what the Russian air force can do; what we want to know is whether it will do it. This may be as much a matter of reading newspapers or lunching the right man as breaking into an office or "turning" an informant in the Kremlin.

Our old-fashioned image of spying was based on the belief that there was something to be found out; for example, that, in James Bond terms, there was some devastating, exotic piece of technology whose secrets we had to discover. That image was born of an age of technological anxiety, the fear that science was moving so fast and on so many fronts that it was out of control. Sputnik, the first space shot, enforced the sense that Western confidence in its own technological superiority might be misplaced. And that was combined with our sense of the Soviet Union as a vast, unknown landscape. After the last war, spy planes had to fly over the country just to map it properly. Spying was an expression of our fear of the profound illegibility of the enemy.

That fear is with us still. But now it is not of the doomsday weapon but rather of the scale and formlessness of the available information and of the uncertainty of who, exactly, is the enemy.

So, even though the rules have changed, we cannot stop playing the great intelligence game. But from now on, it really will be intelligence and the players will be thinkers, not shooters. A good thing, too, you may think, but not necessarily a less frightening one.