Our centuries saw two deep revolutions of physics with far ranging consequences for the way we view the world. The revolution brought about by the Theory of Relativity, or rather the two Theories of Relativity, the Special and the General one, implies that the notions of space and of time are not absolute. Rather, they depend on the relation of the observer to the observed phenomena. This is deeply rooted in Mach's principle: "Physics should only make statements about observable quantities". The clear analysis as provided by Albert Einstein of what it really means to actually measure distances in space and differences of time implies that neither space nor time can be absolute.