We reach all too easily for the word "unimaginable" these days. It is the superlative of choice when it comes to expressing moral shock or consternation, and the recent London bombings - achieved and unachieved - provoked another flurry of sightings. And nine times out of ten the word turns out to be misapplied. It's nearly always the case, for example, when what's proposed as "unimaginable" is the grief of those bereaved by an attack or the terror of those caught up in it.
If only these things were "unimaginable" it would be a good deal easier to obey the new civic duty of normality - but the truth is that we can imagine them all too easily. We might not get close to their actuality - but then that's what imagination means. It's not the real thing, it's a synthetic substitute for immediate sensory experience which allows us to rehearse our reactions at a distance. And it is surely only the most stoical or unimaginative among us who haven't been rehearsing like crazy just recently - startled by the sudden inexplicable jolt on the Tube or looking warily at a passing bus and seeing the panelling explode out sideways in our mind's eye.
Which raises a question about the artistic responses to terror - such as Chris Cleave's recent novel Incendiary - written, according to his own account, as a response to the September 11 attacks. Incendiary was published on 7 July, an uncomfortable coincidence of timing, given its fictional account of a suicide-bomb attack on a football stadium. For some reviewers, this made the book peculiarly timely - for others it rendered it virtually unreadable. But my problem with the book was not so much that it invented a fresh horror when we had more than enough to deal with anyway - but that it chose to imagine what it felt like to be on the receiving end of terror rather than what it felt like to deal it out.
True, the female narrator of the book, whose son and husband have been horribly murdered, engages in a kind of mental conversation with Osama bin Laden - but her rhetorical questions bounce off an impenetrable void. And the novel itself recoils into the obvious - sorrow, incomprehension and loss - the same territory that Jonathan Safran Foer found himself wandering around in his novel about September 11, Extremely Loud and Incredibly Close.
There are things to be said about victimhood - and Cleave touches on one of the more difficult ones when he imagines the mounting death toll from his atrocity. "Maybe it was 16 days after May Day or maybe it was only eight when the death toll finally reached 1,000," he writes, "I think the whole country had been secretly hoping it would get there. It was like a relief when it happened." This, I think, says something uncomfortably true about the way terrorism feeds our ill-considered appetite for the historic and momentous. But that doesn't seem enough to compensate for the larger failure - to take us to a place we can't easily reach under our own steam.
Some writers have taken on this trickier task - including Matthew Kneale in his last linked collection of stories, Small Crimes in an Age of Abundance, which concludes with a bomber's-eye view of an Israeli suicide attack. But they require far more nerve to take the task on, given the popular confusion between imagining a state of mind and approving of it. Actually it would be dishonest not to admit that Kneale's imagining is empathetic - presenting his bomber as a kind of victim himself, of shame at the betrayal of a relative and of the spiritual blandishments of his handlers.
But then any halfway decent art is liable to make our reactions to such events more complicated, not less. And the best art can somehow get you inside an alien mind without dulling the edge of its uncanny difference. Doris Lessing did it in The Good Terrorist, which depicted how seamless idealism and ruthlessness can be. But the matchless model remains Conrad's The Secret Agent - and the "dingy little man" who is at the centre of the novel's bomb plots. "I depend on death, which knows no restraint and cannot be attacked", he says to a colleague, when boasting of his invulnerability and resolve. And in the self-aggrandisement of his words you approach the psychological centre of the suicide bombers - their exultation at the way their ordinariness (which they hate and cannot live with) conceals this secret power. Conrad's terrorist spends all his time searching for "a really intelligent detonator" - one that could account for changed conditions. We now know how easy it is to make one. But writers can still help us understand their inner workings. The consequences of their work we can imagine for ourselves all too well.