For, when all is said and done, it is only the observer who can verify the truthfulness of our assumptions and hypotheses. This central role of the observer is accentuated by referring to spacetime in terms of the observer's frame of reference . This is rather like a mathematician's number line. We shall therefore assume only the following about spacetime. Space and time are homogeneous.
Space is isotropic.
Spacetime is continuous. By the homogeneity of space and time we mean that there is nothing different about either space or time from one place or epoch to another. The asserted isotropy of space adds the further rider that nothing about space distinguishes one direction from another. Continuity of spacetime is meant in the sense that there is no position in space, or instant of time, for which events can't happen. In particular we assert that there is no position in space, or instant in time, at which it is not possible to both emit and receive a flash of light.