Galilean (and Einstein’s) invariance principle gives significance to the relative motion of reference frames (observers). And it states that the laws of physics do not vary in reference frames that are in relative uniform-rectilinear motion and vary if the reference frames are accelerated. Therefore all inertial frames are in uniform-rectilinear motion relative to each other. This paper presents a new principle that gives significance to the relative motion of physical systems (e. g the solar system) and not to reference frames. It shifts the focus from the motion of reference frames (observers) to the motion of the physical systems to be observed. It states that the laws of mechanics and gravity (in their simplest or complex forms) are the same in physical systems that are at rest relative to each other and vary if there is relative motion between the two systems. This implies the validity of absolute reference frames in which the laws of mechanics are in their simplest forms. The laws of mechanics (and gravity) are independent of the choice or relative motion of reference frames. Therefore, we can use any reference frame provided that we know its motion relative to the physical system or relative to an absolute reference, with the same result for all reference frames; however, we can use reference frames attached to the physical systems for convenience (for example to the centre of the sun for the solar system). An observer should not attempt to apply the laws of mechanics before knowing his /her state of motion relative to an absolute reference or relative to the physical system to be observed. The relative motion of physical systems has kinetics effect, whereas the relative motion of reference frames (observers) has only kinematics effect. Inertial frames may be redefined as: All inertial frames are at rest relative to each other. Thus a frame which is in motion relative to an inertial (absolute) frame is not an inertial frame. All real motion has a cause and hence we can differentiate between real and illusionary motions. Motion without a cause is not a real motion. This is obviously an opposing view to relativity. A thought experiment with two identical solar systems and two observers is presented to show that the laws of mechanics and gravity are not the same in physical systems that are in relative motion.