Answer:
Step-by-step explanation:
Suppose you are standing on a platform connected to a train station. You appear to be at rest to a third person looking at you from the parking lot.
But what happens if you are observed by someone in the dining car of the train that just passed you?
You appear to be moving. He can prove it because your relative size is getting smaller. How does he know that it is he that is moving and that you are standing still? If he has no idea what's going on with his motion, he also has no idea who is moving.