Having a clear statement of your main point is an obvious and legitimate way to begin. The danger is that it can become convoluted and abstract. One solution is to make it personal, punchy or provocative.
Life is difficult.
M.Scott Peck, The Road Less Traveled
With only three words Peck flings a bucketful of cold realism into our overly optimistic faces, waking us from our self-help stupor. Part of the power of his opening comes from the contrast it presents to a society committed to the easy life. Losing weight should be as easy as taking a pill. Loving someone should be simple. Learning a language should be as painless as listening to a recording. We just don’t want to hear that all these take time and effort. But once we come to peace with that truth, says Peck, life becomes better.
Even fiction can begin strongly with a very nonfictiony preposition.
All happy families are alike, each unhappy family is unhappy in its own way.
Leo Tolstoy, Anna Karenina
Because no self-respecting novelist would write a story like a lot of others, we know this will be a story about one of those unique, unhappy families.
Were thesis openings the thing to do in the nineteenth century? I don’t know, but they certainly knew now to do it well. Here’s another:
It is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife.
Jane Austen, Pride and Prejudice
And we know just the woman for just such a man – or at least we will shortly?
from “Write Better: A Lifelong Editor on Craft, Art, and Spirituality” by Andrew T. Le Peau, Intervarsity Press