Have you heard the one about "was"?
Finding the first line

The darkroom and the double-helix

Because terror of the blank page and the "wrong" words hamstrings so many writers, I spend a lot of time saying, "Just write. Nothing's set in stone. You can change anything, once you've got words on the page to change". But in the long-gone days of silver halides I learnt that although you can do amazing things in the darkroom with both light and chemistry, you can't print what isn't in the negative. The pattern of dark and light across the film - what was there when you saw and took the photograph, that seemed to say what you wanted to say - is all you have to work with, and will always set the outer limits of the picture you can make.

Similarly, the key to learning to write short fiction - or long fiction or poetry, come to that - is learning to recognise the right size and shape of idea. And I know that there's a limit to what I can teach a student about writing if we only talk about their work in progress. In other words, your idea - your project - will always have its outer limits; there will always be things it can't be or do. I've come up hard against this fundamental fact of creative work in the novel I've spent the last couple of years writing. It's undergone two bouts of major surgery and innumerable minor operations. Everything's working except one utterly crucial thing.

A project is the combination of what you want to write about, and how you want to write it, and the two evolve together: the characters and situations beget structures and voices, the drive to shape a story begets events and ideas for the story. Some writers would schedule another operation, others would write it again from scratch. I know what needs doing, and it can't be done within the outer limits of what this project is. The negative is wrong. When I realised that I lost all my drive to go on trying to make it work: the What of this project is the same thing, now, as the How, and they can't be separated. What would work is a different novel.

So it goes under the bed, and I don't mind at all; it's not wasted, because nothing you write is ever wasted. I've learnt a lot of things you can only learn by pursuing a project to its outer limits, and all those Whats - that time and place, those drives and passions, those dangers, those ideas - are still there, as is all the research. This is nothing new for me: The Mathematics of Love was Stephen Fairhurst's third outing, A Secret Alchemy Elizabeth Woodville's second, but this one was new from the ground up. I won't ever go back to this actual text, but one day I'll know the How and then, thanks to the value of forgetting, the right bits of What will come back to me.

And at last I can embark on the novel which has been clamouring to be taken seriously for months, although it's a huge and rather different project, and I've no idea if I can do it. There are two kinds of project that I can't sustain: the ones that I know I can't make work, and the ones that I know I can. The only drive that's strong enough to keep me going for the long haul of the novel is the need to find out if I can make something work: whether this growing and twisting double-helix of What and How can become a living creature.