Einstein's relativity theory can be understood as a theory of absolutes. The two fundamental principles on which his theory was based both involved invariants in the physical world. First, he argued that the laws of physics do not vary in different inertial frames of reference; they are the same in all non-accelerating frames. Secondly, he argued that the speed of light is constant in all frames of reference. Beyond these two principles, Einstein linked space and time in an invariant space-time that could serve as a metric to measure the distance between events in different frames of reference. Hermann Minkowski, a German mathematician and teacher of Einstein, provided the clearest formulation of this concept of invariant space-time. He argued that space and time are mere facets of an invariant all-encompassing space-time continuum in which one may measure both the spatial distance between two simultaneous events, and the temporal difference between two identically-located events. Although the spatial and temporal coordinates of two events can shift, they are always linked by the same invariant space-time, which serves as an absolute metric, or standard of measurement.