 Wilfrid Sellars  believed that the various invariant or observer-independent elements of Minkowski spacetime (like the light cone structure to be described below) that are typically given primary consideration in treatments of relativity from a spacetime perspective are abstractions from and secondary to the ‘perspectival’ pictures or perspectives, the myriad of inertial systems or reference frames. When it comes to time, however, he believed that there is something even more fundamental than these perspectives: