ii8 THE POPULAR SCIENCE MONTHLY 



consequences. If an impartial revision of deduction then leads to the 

 detection of new consequences which agree with the new facts, such 

 added agreement greatly increases the probability of correctness ad- 

 judged on the previous agreement. More significant still is it, when 

 certain peculiar or complicated consequences are deduced, for which no 

 corresponding facts had been previously discovered, and when a return 

 to the field of observation discovers facts of the peculiar character and 

 in the significant situation assigned to them by deduction. Tliis gives 

 wonderful strength to the hypothesis from which consequences so 

 prophetic can be derived: indeed, evidence of this is usually regarded 

 as convincing, for the possibility of such a degree of accordance of 

 consequence and fact being the work of chance is practically ruled out. 

 Finally, if in the course of years, many investigators find many com- 

 plicated facts in many parts of the world, all of which are successfully 

 matched by the elaborate consequences of an hypothesis that was in- 

 vented long before observation was so widely extended, the probability 

 of correctness rises to so high an order that the truth of the hypothesis 

 may be accepted, and it may be promoted to the rank of an established 

 theory. The unseen facts that such a theory reveals are commonly ac- 

 cepted as of an equal degree of verity with the facts of direct observa- 

 tion. 



The will or the wish of the sane investigator has no power to with- 

 hold belief, when this stage of theorizing is reached. And yet it can 

 not be too carefully borne in mind that even if all the above require- 

 ments are satisfied, the most that can be said for the established theory 

 is that its probability of correctness is so high that its chance of error 

 may be disregarded. The fair-minded Playfair phrased this aspect of 

 our problem admirably a hundred years ago in the case of river valleys : 



Every river appears to consist of a main trunk, fed from a variety of 

 branches, each running in a valley proportioned to its size, and all of them 

 together forming a system of vallies, communicating with one another, and 

 having such a nice adjustment of their declivities, that none of them join the 

 principal valley either on too high or too low a level; a circumstance which 

 would be infinitely improbable if each of these vallies were not the work of 

 the stream that flows in it.* 



It is particularly in this matter of the increasing probability of cor- 

 rectness that the nature of geological or geographical proof is so unlike 

 that of geometrical proof. There is never any talk of increasing the 

 probable correctness of a geometrical theorem, when several different 

 demonstrations are given for it. Each demonstration is absolutely 

 correct alone, as far as anything can be absolute in the limited experi- 

 ence of our finite minds. But in our subject, it is always appropriate 



• J. Playfair, " Illustrations of the Huttonian Theory of the Earth," Edin- 

 burgh, 1802, 102. 



