Leibniz stated: "Thus committed to maintaining that if there were nothing more to motion than relative change of position, then, since motion could be ascribed with equal right to, say, Train A or Train B."
How does Leibniz's observation behave when one introduces a boundary condition? One can start thinking of bodies with respect to the boundary. Or even more interesting the boundary relative to itself. Have these thoughts been explored by philosophers? (Would be an interesting read in the context of general relativity)
P.S: I asked something similar previously