Oh, it was so interesting to read Mach's books on mechanics and his ideas, his influence on physics. He was an utter skeptic on atoms. He thought that atoms were a creation of insane intellect, but mechanics, why should a particle put up a resistance to being accelerated, accelerated with respect to what? Well, Newton said Absolute Space, and for Ernst Mach absolute space was nonsense. If a particle put up resistance to being accelerated, it's accelerated not with respect to absolute space, but with respect to the matter out there in space. So inertia in his view of mechanics, inertia here arises from mass there out in space. That's such a simple slogan, but how to see it? And... maybe I can tell a little story about Kip Thorne and Charlie Misner and I taking a twenty minute break once while we were writing our book. We were using by the sufferance of the institute at that time, a room in the Institute for Advanced Study, and so I said "Let's walk around the corner and call on Gödel." So neither of them had met Gödel, so I knocked on the door. It was a glass door which looked out over a pond and trees beyond. A beautiful warm spring day, but from the glass door we could see inside Gödel with an overcoat buttoned up around his neck and an electric heater going on the floor.