My wife Jennifer and I were at dinner the other night with good friends, at their place, high on the hill (local mountain). Dinner was fine (organic chicken, roast veggies, creme brule). And the conversation, like the wine, did not stint.
We were talking about books, and our hosts asked what we thought was the theme of a book? Books in general? I inquired. Particular books?
They tend to read non-fiction, earnest non-fiction, I might say, and are practicing Christians, as Jen and I were once upon a time.
My first thought, as a creative writer, is that we don’t start with themes. Poets especially start with an image, a sound, a story — and go from there. We reason, if you call it reason, inductively, going from particular observations to ends, themes, ideas.
In fact, these ends or themes may not matter much at all to poets, especially if they’re writing short poems. They may be unconscious or semi-conscious at most, left to critics and other analysts to ponder. In my own practice, I think of only one long work, just completed, that might have started deductively: a 31-part poem about brain cancer, pain and suffering.

For the most part, even novels start, I suggest, with a story or a character, not a firm idea or theme. I affirm Mark Twain’s comic take, at the beginning of Huck Finn, on the business of sleuthing a work of art and being a critic:
Notice: Persons attempting to find a motive in this narrative will be prosecuted; persons attempting to find a moral in it will be banished; persons attempting to find a plot in it will be shot. By order of the author, Per G.G., Chief of Ordnance.