xliv Trans. Acad. Sci. of St. Louis. 



The disagreement in the observed and theoretical motion of the 

 Moon has led to an attempted modification of the measure of time, 

 and to the substitution of an inertial time gaining nine seconds 

 a century on the mean solar time. This would remove the difference 

 in the Moon's longitude. 



The truth of A is established through experience quite inde- 

 pendently of B, and so far as a representation of the behavior of the 

 solar system throughout historic time is concerned, the Postulate of 

 Contingence is sufficient. What then is the positive content of the 

 postulate of causality which makes its retention essential to the 

 existence of science? It is this: 



The Postulate of Causality builds the program according to which 

 we must envisage the geologic past, and prescribes the confines 

 within which expectation places the future. 



Without it neither would exist for us. Affirming B is equivalent 

 to postulating the existence of a geologic past and a future accessible 

 to us through a physical theory satisfying the requirements of A. 

 The present planetary theory adequately represents the behavior of 

 the solar system throughout historic time, but extended into the 

 geologic past, it pictures a system without observational control. 

 The past is constructed on the assumption of a permanent planetary 

 theory, and if this theory does not possess permanence the past cer- 

 tainly furnished no means of detecting it. 



Once the Postulate of Causality is replaced by a mere contingent 

 coherence of phenomena, the past ceases to be and the mental horizon 

 lies entirely in the world of experience. The data of that world 

 may be ordered in any way that fits it, and the complex of rela- 

 tions connecting such data is controlled solely by the resulting errors. 

 The problem is to make these as small as possible by continuous ad- 

 justment, and experience itself furnished no ground for belief that 

 these outstanding errors themselves constitute an ordered group. 

 They stand always, a chaotic totality, between the physical theory 

 and the world of experience, preventing us from regarding that 

 world as subject to law in the sense that relations are rigorously 

 satisfied by its phenomena. Suppose then that we replace this world 

 of experience by a fictitious world governed by a causal principle and 

 osculating the world of experience during the present. This fictitious 

 world will possess both a past and a future rigorously reducible from 

 its present, and the postulate of causality identifies the past and the 

 future of this fictitious world with the past and future of the world 

 of experience, and so creates both for the world of experience. 



Does a past or future created in accordance with the Postulate of 

 Causality possess reality? The older point of view, which regarded 

 empirical verification as a proof of reality, which nevertheless did 

 not cease to exist even when all connection between the external 

 world and its representation was broken, has given way to a mod- 

 ern conception of reality of which invariance is the criterion, but re- 

 gards this invariance as relative and approximate. 



