 Most of the rest of the book is a restatement of the Axiomatization with more extended examples and fewer proofs, and explicitly construed as a reduction of space time relations to causal relations supplemented with definitions. The fundamental causal relation to which Reichenbach appeals must be asymmetric and allow reidentification of objects or their features. Reichenbach thinks to obtain the asymmetry via a criterion of causation that invokes both the preservation of something material and the asymmetry of intervention. A “mark” placed at the beginning of a causal process can be received at the end of the process; Reichenbach’s example is the color of a light signal. The asymmetry is that if the beginning of the process is marked, the end of the process bears the mark, but if the end of the process is marked, the beginning does not bear the mark. The analysis thus depends on a distinction between observing a mark and the asymmetric relation of causing a mark. Reichenbach recognizes that for his empiricist, causal construction of the space metric, signals must be reidentifiable, a relation he notes that Kurt Lewin had discussed (in his 1922 Habilitationsschrift) as “genidentity” (see also Padovani (2013)). Unsurprisingly, Reichenbach offers no guide to reidentification, which is obtained in all cases, including those of light signals, from elaborate convictions—or at least habits—about all kinds of details and regularities of the world. Had Reichenbach thought it necessary to his constructions in the Axiomatization first to establish empiricist criteria of genidentity, the construction would not have begun. 