I don't know about you, but I can't imagine writing a novel which was trying to set forward a thesis, or prove a point. Indeed, when I told a literary journalist that one of the themes of The Mathematics of Love turned out to be lost children and she asked me what it says about lost children, I floundered: I hadn't had an argument or a thesis, just an emotional centre for the novel.
But the novel I've just finished is the first which has come from an idea. I knew from the first moment that it was going to be about betrayal. So I went what Ishiguro calls "location hunting": I sought out a time and a place, and then people, which would embody all the things I wanted to... I nearly said "say", about betrayal, but that sounds too tidy and conclusive. "Explore"? "Evoke"? "Embody", perhaps, because that's what we do, as novelists, isn't it: we tell stories and poke ideas around by means of particular, fictional human bodies. These bodies are enmeshed as we all are in nets of allegiance and trust, and now they're threatened.
The novel I haven't started writing is the first thing I've ever written which takes the least account of a review of my work; in The Times Sarah Vine said, of The Mathematics of Love, "Everyone is, at the core, vulnerable, their happiness bittersweet and fleeting but nevertheless priceless." And when I happened to remember that (it's the sort of review you do remember) the wisps of story and characters floating around in my mind suddenly made sense, and the novel got its word; this is a story about happiness.
So then I started thinking about whether there was a one-word description for their predecessors. I knew from early on that The Mathematics of Love was about voyeurism, and later lost children. But if you'd asked me what A Secret Alchemy was about I would have said "Elizabeth and Anthony Woodville, and a modern historian." It was my agent who said, "It's about marriage, isn't it?" and she was utterly right, and the title came from that knowledge.
So could you give your novel a - I hesitate to call it a theme - a one-word tag? It's a bit terse for the inevitable interviewer's question, "What's your book about?" but at least it'll stop you rabbitting on for paragraphs: the interviewer wants two sentences, max. Like an elevator pitch, the idea of reducing your book to a single word is at once horrifying, because it seems so reductive, and immensely useful. Not because it's easier to sell: one word can't be High Concept in the way the movie industry wants. It's useful because, your one word having emerged or been found, it stays in your ear as you write. Pieces of plot become coherent and significant, instead of being a means of pushing your characters around. Small scenes, minor characters become part of the whole when your one word colours what they say and do. There's a reason to choose one name, or refer to (not quote - too expensive) one song lyric, rather than another. It's a way of channelling some of the millions decisions you must make into a more coherent stream than they'd otherwise be. It's not reductive, any more than it's reductive to find the first right colour for a room in a precious ornament or beloved rug: it's round that which you'll build the rest of the colour scheme. Even though only some people will look at it and say, 'That's a wonderful colour!", just about everyone will (you hope) find the room more welcoming/appealing/ordered/attractive/exciting, because of it.