Mart

What does an apple (you) have to do to know that it is moving at 1m/s. Remember, there is no absolute velocity.

What do X and Y have to do to know their individual velocities?

You say one MUST be moving faster than the other. What do X and Y have to do in order to determine their relative velocity?

X and Y are assumed to be moving at a steady non-zero velocity relative to each other (first line of post). If both X and Y are stationary then how can the distance between them be changing steadily? No changing distance = zero relative velocity. That's the problem.

Imagine X and Y moving at a steady non-zero velocity relative to each other. X might claim that he is stationary and that Y is moving. Y could reverse the claim. We scratch our heads and say that provided we stick to steady motion we do not know any way of distinguishing between moving and stationary. But this raises a problem. Although a huge number of different claims about the velocities of X and Y can be made (none, apparently, provable) it can't be the case that both X and Y are stationary. The steady non-zero velocity they experience relative to each other would vanish. It has to be true that at least one of them is moving. Is there a solution?