Someone I know wanted to start writing her book before she'd worked out what would happen at the end. She had the first half of the story, but nothing in the second half. Should she start writing? She seemed confident that by the time she got to the middle she'd be able to muddle her way through. She'd find out what happened in the rest of the story as she went along, she said.

I know some people do this. Some of them are even pretty successful. But it's a terrible idea. If you do this, no mat...