Abstract

Hans Reichenbach (1891-1953) was a representative Logical Empiricist philosopher who systematically developed the philosophy of space and time through philosophical analysis of the theory of relativity. His best-known work on the philosophy of space and time is The Philosophy of Space and Time (1928), but his philosophy is actually concentrated in his previously published work, Axiomatization of Relativity (1924). At about this time, Reichenbach's philosophical analysis of space and time in the relativity theory came to an end. Reichenbach's philosophical conclusion was that the order of space is reducible to the order of time, and the order of time is reducible to the order of causality, and that even if the metrical relationship between space and time changes, the topological and causal relationship remains constant. Eventually, he arrived at a causal theory of time, but he still could not explain why time continuously flows from the past to the present and into the future within the scope of relativity theory. Therefore, he attempts to show that the flow of time actually exists by distinguishing between the future and the past based on an empiricist perspective. The paper containing his attempt is “The causal structure of the world and the difference between the past and the future.” There are two key insights from this paper. First, the past is determined in the sense that the cause can be inferred from only the part of the effect, but the future is not determined in the sense that the effect cannot be inferred from the part of the cause alone. Second, the difference between the past and the future can be objectively proven by specifically calculating and showing that the postdictive probability is equal to or greater than the predictive probability derived from events in a simultaneous cross-section. If Reichenbach's core insight is correct, we can objectively prove the reality of the flow of time leading to the past, present, and future experienced by humans while excluding subjective elements included in human experience as much as possible. In addition, Reichenbach's philosophical perspective can give important implications to the philosophical discussions on the ‘reality of time’ that are actively taking place today.