It is believed that...

No fixed point in the universe. We must be anchored somewhere.

There is no heavenly body in our universe that we can use as a stationary reference point.

No matter what we may choose as fixed points, we cannot help operating with certain entities which have no absolute significance.

As Newton pointed put long ago, it is impossible to tell whether or not a ship is moving through the water by any experiment performed inside the ship.

Mathematically speaking,

The problem is this. We have the right to set up coordinate axes with the help of which we can orient ourselves in space. We establish a Cartesian frame of axes or a Cartesian reference system. But there is an infinite variety of such systems. We cannot find an absolute center in space, nor are there preferential directions in space which would decide for us how we should orient our coordinate axes. We can set up an infinite variety of coordinate systems by putting the center of the reference system at any point in space and orienting our coordinate axes in any way we like, although keeping them mutually orthogonal. This means that the same physical event can be viewed from an infinity of reference systems, all of them being equally suited to the task of serving as a possible framework for our physical measurements.

While we can certainly observe the same physical phenomenon from an infinity of reference systems and thus obtain an infinity of different measurements, it is nevertheless unthinkable that the physical law itself should be influenced by the fact that the observer is in this or that frame of reference.