Time, like space, is not an ideal entity of a Platonic existence perceived by an act of vision, or a subjective form of order imposed upon the world by the human observer, as Kant believed. The human mind is capable of conceiving of different systems of time order .... Among this plurality of possible systems, the selection of the time order that holds for our world is an empirical matter. Time order formulates a general property of the universe in which we live; time is real in the sense space is real, and our knowledge of time is not a priori, but the result of observation. The determination of the actual structure of time is a chapter of physics – this is the result of the philosophy of time. Reichenbach, 1959,p. 155