" I get into trouble for saying that ‘Al Qaeda’ doesn’t exist, but there is no such organisation."
Peter Hitchens, Mail on Sunday, 10 July 2005

The intelligentsia have never been keen on the “war on terror”, often for good reasons. Perhaps what irritates them the most is how the rhetoric of the campaign has been so simplistic, reducing complex situations to straightforward battles between good and evil, and blurring important distinctions between Saddam Hussain and Osama Bin Laden.

This justified scepticism, however, has led some to be too quick to dismiss every claim made by the American and British governments. Both have been accused of exaggerating the terrorist threat, sometimes by the same people who, when Madrid and then London were both hit, claimed that these attacks were inevitable.

The most knowing of these sceptical views is that Al Qaeda does not even exist. But what does this claim boil down to? What it means is that Al Qaeda is not a single, centrally-run, global terrorist organisation. The attacks in New York, Madrid and London were not all planned and directed by Osama Bin Laden and his team. Rather, autonomous cells, which may have had no direct contact with Bin Laden’s group at all, organised these attacks on their own.

To conclude, however, that Al Qaeda therefore does not exist is as premature as saying that, for example, we cannot talk about the French Resistance or Italian Partisans during the Second World War. These resistance movements could not be tight, highly-centralised movements with strict hierarchies. Rather, local groups often had to work more or less on their own. However, their shared goals and recognition of some kind of leadership meant it is often proper to talk about both movements as single entities.

Of course, it is possible to be confused by the language and to assume that because we use singular nouns there must be singular, distinct entities they refer to. But it is our mistake if we do so, because nothing in logic or language demands that everything we talk of as a single entity is a simple, unified object.

Gilbert Ryle argued that this false assumption was an example of what he called category mistakes. These occur when we think of one thing as though it were another kind of thing, and so misunderstand its nature. One of his most vivid examples is of the tourist who demands to see Oxford University and is then puzzled when he is shown only colleges and libraries. He wanted to see the university, thinking it was a single building, not realising that the university was nothing more than the sum of its constituent parts.

There are many things that clearly do exist, yet cannot be pinned down to single entities or formal organisations. The anti-capitalist movement is one example. It is quite right to say that the anti-capitalist movement does not exist as a coherent political entity, but rash to say it therefore doesn’t exist at all. Likewise, many belief systems exist even in the absence of formal statements of doctrine or central organising bodies.

That is why it is wrong to say that Al Qaeda cannot exist in the absence of a global command and control system, with Bin Laden at the top and individual terrorist cells at the bottom. For Al Qaeda to exist, all it needs to be is a general movement with common aims, which takes inspiration and guidance from certain leaders, such as Bin Laden. It needn’t even formally share resources, tactics and information, although terrorist groups do this via the internet, whether they know the information providers or not.

Does such a movement exist? Peter Hitchens, taking his cue from Jason Burke’s widely admired Al Qaeda: The True Story of Radical Islam, thinks not. Burke, however, did not write a book about something that doesn’t exist. Rather, as one reviewer put it, Burke’s real thesis is that “Al Qaeda as we know it does not exist” but that it is a “’formula system’ for terrorism, an exportable praxis”. Hitchens therefore goes further than Burke when he says there are only many groups “vaguely linked by a common ideology”. For all I know, Hitchens may be right. It would not be right, however, to claim that the lack of formal links between these disparate groups by itself shows Al Qaeda is a figment of the intelligence services’ imaginations. Too many mistakes have already been made in the so-called war on terror: category mistakes should not be added to them.

Julian Baggini’s new book, The Pig That Wants to Be Eaten and 99 Other Thought Experiments, is published by Granta.