What does it take to write a book of potentially publishable quality?

I have no idea...no, really, I was asked this in a email a few days ago, by a student on an MA in creative writing, and I'll answer it again because I need reminding occasionally too. I've met a lot of people who are aiming to write a novel, and they seem to fall into three camps.

• Those who think they have a novel in them but haven't started one because it seems so unlikely that they would be able to get published there's no point. My advice would be that the reason most of us write novels is because we are driven to write anyway, publication isn't the only goal. Write short stories, send them off to magazines, put your toe in the publishing pool.
• Those who have written a novel and are now polishing and honing the masterpiece prior to taking the marketplace by storm. My advice would be to remind them that first novels very rarely get published,  most of us learn a lot with each novel, and statistically we are more likely to get novel 4 published than 1, 2 or 3. Put the book out there, but start book 2. And book 3. Grieve, move on, get stronger.
• Those who have written a stack of books but can't finish them (oh, that's me!). Take a long, hard look at the best of those books, then rewrite to make the book stronger. Whatever the core of the book is, whether it's a relationship, a threat, a journey - make it more so. If it's a tense thriller, make it more tense. If it's emotionally draining, make it more so. Rewrite, rewrite. If you aren't sure you are making the book better, join a writing group, take a class, apply to an MA, work on your craft. Read good books in your genre, analyse how they do it.

And remember, no matter how good your query letter and synopsis, it's the book that needs to impress the reader. Make the first three chapters absolutely perfect, good enough to enter a competition, say. Then rewrite all the other chapters to be at least as good. If there's one thing I've learned in the last few years and the five books I've written over them, it's that writing is hard work, and your first draft will be enormously improved by being rewritten and re-crafted, probably several times. Whether you do that as you go along, or through successive drafts, those words can probably be improved. I don't know whether I can get a book published, but I know I've got pretty close over the last few months. I'm waiting to hear back from editors, but I'm not sitting around. I'm off to write more books, and more drafts, to make my chances even better. And remember...