Risu nodded. "That's it exactly, Mr Leone. Well done." He addressed the rest of the class. "I'm sure everyone reached the same conclusion as your classmate here. As I said, the situation is rather straightforward. For person A, an object might be moving. For person B, the same object might appear motionless. It depends on their relative perspectives.

"However," he continued, wandering up and down, "here's where it begins to get tricky. There is an unwelcome implication to this loss of absolute motion. I wouldn't expect anyone of you to spot it, so I'll help you out: We have to get rid of the idea of absolute space. Einsteinian relativity does this absolutely ruthlessly, but even Galileo seemed to have realised it."

Risu paused briefly. "Loss of absolute motion means loss of absolute space. There is no way to define where you are exactly." He looked at the students seriously. "In the same way you can only define motion or stillness relative to something else, you can only say where you are relative to something else. Now, does anyone have any idea how Galileo might have come to that conclusion? This is a very difficult question, but perhaps someone might be able to figure it out? Perhaps this hint will help: If there was absolute space, we could easily decide if something was actually moving or standing still. We could say, to recap our example, whether the book on the train was moving or not with complete certainty, and every observer would agree on the same thing."