  As far as kinematics is concerned, Leibniz, like most of his  contemporaries, accepted the observational under-determination of  constant linear motion. That is to say, he granted that – if we bracket considerations of force – there’s no saying which of  two bodies moving relative to each other with a constant velocity is  really moving. So, for example, Leibniz would have conceded  that we can’t tell just by looking whether Train A or  Train B is really moving, even if they are moving  with a constant velocity relative to one another. This observational  under-determination or “invariance” – often called  Galilean invariance – is still accepted today,  although the natural assumption that it makes sense to speak of any  body as “really” moving with a constant velocity  independently of an arbitrarily chosen frame of reference is not. 