Those out are generally finishing up football games, or walk with that focussed expression of the homeward bound: they've reached their halfway mark and are on their way back.
Walking is the same thing as good thinking. There's movement, yes, but the pace is such that you get to track your own progress: you don't get there without remembering how you got there. It's the opposite of bad thinking, which is when you ponder and confirm the views you reached unaware.
Tonight, as often before, I walked pondering the book I'm writing. When I reached my halfway point I identified a problem I'd been circling for some days, but which I hadn't managed to define - to my surprise, it turned out to be a technical problem to do with why my main character would want to travel from London to Zambia. I thought to myself, 'Yes, of course, I just have to...' and so on.
In the morning, I can make the change, and my character will get to board his flight to Lusaka.
In week one of my teaching semester, I ask my students to write a hundred words on the topic, 'Why I Write'. George Orwell wrote to this title once, and we use his piece as stimulus for ars poetica that students write and then can revisit at the end of semester, perhaps even at the end of their degrees.
In a way, it's a hopelessly broad topic. And yet even if it's impossible to define, and even if the response changes often over time, I think it's still worth wondering.
My answer tonight: I write because of moments like this evening, when I am witness to a conversation between my understanding of a project and the task of writing the next section. It's a sort of instruction, from one side of myself to another, that I would call inspiration.