BOOKS / Second Thoughts: All in the mind of a plumber's lonely son: Patrick McGrath on the flash of light that led to the psychotic Spider (Penguin pounds 5.99)

THE NOVEL began with a knock on the door. A man answers the door. He gasps with horror. Standing there is a woman supposed to be dead these last 10 years. Who is she? How did she 'die'? The story unfolded in my imagination in a flash, in a few seconds, no more. And that was the genesis of the novel, that short bit of narrative wire that over the next 16 months got pulled and bent and twisted into the delusional self-portrait of a matricidal schizophrenic.

Actually that's not how it began. What came first were Bill Brandt's spooky photographs of the East End in the 1930s, gloomy atmospheric images of alleyways with lamplight spilling over damp cobblestones, tacky pubs with stout barmaids grimly pulling pints, cramped houses, bad teeth, the working Thames, furtive sex. Somewhere in all this, I knew, lay a novel. Only then was that knock on the door possible.

First draft was writing out the story, in the course of which the dead woman at the front door somehow disappeared. But an infidelity theme announced itself loudly, and the man who opened the door turned out to be Horace Cleg, an adulterous plumber. At about page 150 the question of just precisely who was telling the story became impossible to defer any longer. After a few false starts Horace's boy, Dennis, who witnessed the whole ghastly mess, was chosen.

Now the fun really began. Second draft: Dennis - nicknamed Spider - was to recall the events from the vantage point of maturity, some 20 years later. Who was he, this son of a plumber? One day in the Mile End Road I saw a threadbare character who resembled Samuel Beckett shuffling over a zebra crossing. I followed him for 10 minutes and knew I had my Spider.

Then the idea occurred: what if it's all false memory? What if he's got it all wrong? From there it was a short hop to making him schizophrenic, and from that decision emerged the ambition to depict, from the inside, the experience of psychosis.

Now the fun really began. I reached for my R D Laing. Not the least of the problems was staying faithful to the chaos of madness while providing the coherence and continuity that the intelligible novel demands. Spider's story was necessarily ambiguous, unreliable and confused, but it hinted constantly at a 'true' version of events buried somewhere within its web-work of denial and hallucination. The question was, would the reader be agreeably stimulated by all this, or give up in frustration, or see through it in a minute and toss the book at the wall?

I encountered all three reactions from the critics. (People one talks to tend not to mention the work unless they loved it.) Most, happily, seemed to have enjoyed being tantalised by the uncertainty of never knowing when Spider was telling the truth. And most were generous enough to recognise that it's a delicate technical challenge, when doing an unreliable narrator, to calibrate the obscurity just right. What makes it so dodgy is that after living in someone else's story for months on end, you can't tell what it looks like from the outside anymore. That's in part what editors are for.