I recently read a book, or much of a book, called The Philosophy of Physics , by Tim Maudlin. I say "much of it" because, quite frankly, I couldn't follow it and gave up on it well before the end. But there were parts that I believe I did understand, and on the basis thereof I can say that this was a good book to read so soon after reading Ratcliffe. Ratcliffe, as you may remember, had a quite idiosyncratic take on Euclidean and non-Euclidean geometries. It was an eccentricity that for me rather vitiated his efforts to contribute to cosmology. Anyway: back to M audlin. In one portion of the book he considers at some length the "twins paradox," a puzzle arising from, and often commented upon in the context of, the theory of relativity. Here it is in my understanding, not his. An astronaut takes off for a distant spaceport, travels at near the speed of light, so time slows. Comes back. He is now only, say, 1 year older than he was when he ...