It follows therefore that there are no absolute positions. Every position is determined by its relation to other positions. One and the same point may be either center, pole, zenith or nadir—depending entirely on the point from which it is observed (respectu diversorum). There can therefore be no absolute motion and no absolute time. The ancients based their theory of absolute time on the absolute regularity of the motions of the fixed stars; but since the motions observed from any particular star differ from that of another star there are as many times as there are stars. And, finally, the traditional theory of absolute heaviness and lightness is likewise an error; its tenability was based on the presupposition of an absolute center of the universe. Heaviness and lightness must therefore be understood with reference to the various world-bodies. Sun particles are heavy in relation to the sun, earth particles in relation to the earth. According to Bruno, heaviness is the expression of a natural impulse within the parts to return to the greater whole to which they belong.
The principle of relativity is closely connected with the theory that nature is everywhere essentially the same. We can infer the conditions in other parts of the universe from the conditions about us here on the earth. We observe e.g. that ships, when seen at a distance, appear to be motionless, whilst as a matter of fact they are moving very rapidly, and thus by analogy we may assume that the fixed stars appear to be motionless by reason of their great distance from us. There is no justification for maintaining the fixity of the firmament dogmatically as the ancients and even Copernicus had done.
Bruno therefore challenged the dogmatic principles which Copernicus had still accepted. He saw very clearly
