THE LATEST STAGE 273 



on the definitions. How far this ideal world agrees in 

 substance and in fact with the real world of sense- 

 perception is an affair of experiment. Two and two 

 always are four, whether the two and two be apples, 

 or poets, or cats. Here the definitions are made to 

 agree accurately with the sense-perceptions. But 

 how far astronomical sense-perceptions will agree 

 with the deductions of the Newtonian scheme is purely 

 a matter of observation in the present and probability 

 in the past and future. The fact that concordance 

 exists over the period of two centuries during which 

 our observations extend gives us confidence in the 

 probability that we may extend our predictions back- 

 ward into the past and calculate the dates of ancient 

 eclipses, and forward into the future. But the farther 

 we go, the less the probability of concordance becomes. 

 To be of practical use, all our conceptual schemes 

 must thus enable us to predict the future behaviour 

 of our perceptions. What ground have we for con- 

 fidence that they will do so ? It is simply, as we have 

 said, an affair of probability. There is no certainty 

 in natural science. Because we have known the sun 

 rise for ten thousand yesterdays, we frame a concep- 

 tual sun which rises regularly each morning indefinitely, 

 and, as a matter of practice, we may bet ten thousand 

 to one that the perceptual sun also will rise to-morrow. 

 Because Newton's theory of gravity has met every 

 demand on it for two hundred years, we have great 

 confidence that the complicated conceptual astronomy 

 founded on it by mathematical logic will still continue 

 to agree with our observations of nature that is, with 

 our sense-perceptions. Because all chemical elements 

 proved immutable for a hundred years, we assumed 



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