6 INTRODUCTORY [ch. 



real and physical causes," and has brought it about that "the 

 search of the physical cause hath been neglected and passed in 

 silence." So long and so far as "fortuitous variation*" and the 

 "survival of the fittest" remain engrained as fundamental and 

 satisfactory hypotheses in the philosophy of biology, so long will 

 these "satisfactory and specious causes" tend to stay "severe and 

 diligent inquiry," "to the great arrest and prejudice of future 

 discovery." 



The difficulties which surround the concept of active or " real " 

 causation, in Bacon's sense of the word, difficulties of which 

 Hume and Locke and Aristotle were little aware, need 'scarcely 

 hinder us in our physical enquiry. As students of mathematical 

 and of empirical physics, we are content to deal with those ante- 

 cedents, or concomitants, of our phenomena, without which the 

 phenomenon does not occur, — with causes, in short, which, aliae 

 ex aliis aftae et necessitate nexae, are no more, and no less, than 

 conditions sine qua non. Our purpose is still adequately fulfilled : 

 inasmuch as we are still enabled to correlate, and to equate, our 

 particular phenomena with more and ever more of the physical 

 phenomena around, and so to weave a web of connection and 

 interdependence which shall serve our turn, though the meta- 

 physician withhold from that interdependence the title of causality. 

 We come in touch with what the schoolmen called a ratio 

 cognoscendi, though the true ratio efficiendi is still enwrapped in 

 many mysteries. And so handled, the quest of physical causes 

 merges with another great Aristotelian theme, — the search for 

 relations between things apparently disconnected, and for " simili- 

 tude in things to common view unlike." Newton did not shew 

 the cause of the apple falling, but he shewed a similitude between 

 the apple and the stars. 



Moreover, the naturahst and the physicist will continue to 

 speak of "causes," just as of old, though it may be with some 

 mental reservations : for, as a French philosopher said, in a 

 kindred difficulty: "ce sont la des manieres de s'exprimer, 



* The reader will understand that I speak, not of the "severe and diligent 

 inquiry" of variation or of "fortuity," but merely of the easy assumption that 

 these phenomena are a sufficient basis on which to rest, with the all-powerful 

 help of natural selection, a theory of definite and progressive evolution. 



