COSMIC PHILOSOPHY 



the evidence from the physical condition of the 

 surfaces of Mercury and Venus, Uranus and 

 Neptune, and the moons of the four outer plan- 

 ets, is not yet forthcoming. It would be as- 

 serting too much, therefore, to assert that the 

 nebular hypothesis is completely verified, like 

 the hypothesis of gravitation. But on the other 

 hand, they understand little of the logic of sci- 

 entific inquiry who expect to obtain the same 

 kind and degree of evidence in the former case 

 as in the latter. It was part of Newton's rare 

 good fortune that his hypothesis was the gener- 

 alization of a physical property of matter, which 

 could be verified by a single crucial instance. In 

 none of the concrete sciences can such kind of 

 verification be looked for. A theory relating to 

 a heterogeneous assemblage of concrete pheno- 

 mena can only be verified gradually, as the suc- 

 cessive groups of phenomena in question are 

 one after another successfully studied and inter- 

 preted. Thus the complete verification of the 

 nebular hypothesis, as applied merely to the 

 solar system, involves the complete explanation 

 of the chief dynamic and physical features of 

 the system ; and for this we have yet to wait. 

 Meanwhile the theory possesses such unmis- 

 takable marks of genuineness, it conforms in so 

 many and various ways to the test of reconciling 

 the order of conceptions with the order of phe- 

 nomena, that no one capable of estimating sci- 

 292 



