Consider 2 cars – A and B – travelling in parallel along a straight line along a straight road. A is travelling at say, 40 km/h and B at say, 20 km/h. The velocity of B relative to A is -20 km/h i.e. receding at a speed of 20 km/h parallelly along the straight line. If B was travelling at some angle with the straight line, along a straight line, the motion of B from the reference frame of A would be some straight line at some angle with our original straight line (the road). Now, the speed of B changes with respect to A in both the cases when you step into the reference frame of A. The magnitude and direction of velocity changes but the qualitative nature of the motion remains the same – motion along a straight line. It is not say, that in the earth’s reference frame, B is travelling in a straight line and in the reference frame of A, the motion of B becomes along a curve or something – (as long as the 2 entities – A and B – are undergoing the same nature of motion).
Consider another example – Suppose there are 2 people A and B. And A gives a ball to B. From the earth-frame, A is losing possession of the ball and B is gaining possession of the ball. In the ball’s reference frame, A is going away from the ball and B is coming to take the ball. Now, the first observation in both the cases, i.e. A losing possession of the ball and A going away from the ball are qualitatively the same. And the second observations, i.e. B gaining possession of the ball and B coming to take the ball are again qualitatively the same. So, in both the above examples, the truth is preserved. The qualitative nature of the phenomenon remains the same, irrespective of changing your reference frame from one to some other, in case of a system of 2 “equivalent”-entities (2 cars travelling along a straight line (or both along a curve) or 2 human beings, with a transfer of a thing from one to another).
But now consider 2 minds – A and B. A hits B and derives pleasure from that (say, he is a boxer who enjoys punching people). From the reference frame of the MIND of A, the event is a pleasurable event, whereas say, from the reference frame of the MIND of a third person like you or me observing this, this is an annoying or a surprising event. Now the same phenomenon is enjoyable from one mental reference frame and annoying or surprising from another. These 2 observations/experiences are qualitatively different from each other. The basic nature of the phenomenon experienced changes with change of reference frame. Hence the essential qualitative truth is not preserved across mental reference frames.
This shows that physical reference frames and mental reference frames are fundamentally different. In this case, physical relativity is quantitative whereas mental relativity could be qualitative as well. Hence the very concept of a mind seems fundamentally altered from that of physicality (a physical body).