The Principle of Relativity
There is no such thing as absolute motion, probably. We can only measure our motion relative to other objects. We are used to thinking of objects having speeds or velocities, but these are usually measured relative to the earth,
The diagram above shows man and spaceship in relative motion, and relative is significant here. In space, far from all points of reference, it is only relative speed that is important. The man and the occupants of the spaceship cannot, by performing any physical experiment, determine whether it they or the other that is moving. One could be moving, or the other, or both, as long as their relative velocity is v. This implies that any law of physics must be expressed in the same form in both frames, with measured quantities referred to each reference frame and any physical constants having the same value.
For example, the Law of Gravity could be expressed asin O and in O' withtaking the same value in both inertial frames (subject to both observers using the same units).
This fact gives us confidence to apply the Laws of Physics to distant parts of the Universe and assume their applicability in those parts.