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THE AMERICAN NATURALIST 



[Vol. XLIX 



We hold as a justified demand of the theory of knowledge 

 that every hypothesis must be fruitful ; that is, it must give a 

 number of deductions that can be verified empirically. Every 

 hypothesis which permits us a prediction is to be considered a 

 step in the progress of knowledge, until such time as it is re- 

 placed by a new one, more suitable or more fruitful. 



Biological, and particularly embryological, investigation needs 

 sometimes to introduce as a hypothesis for the explanation of 

 certain empirical facts the idea of so-called "immaterial" (or 

 in Jennings's terminology "non-perceptual") factors. 



This is the chief point on which are based the recriminations 

 of most critics of vitalism, especially of Driesch's vitalism. 



The belief in such "non-perceptual" factors is in Jennings's 

 mind synonymous with obscurantism or dogmatism. To Hitter 

 "the vitalism ... is the belief that organic phenomena can not 

 be fully explained by referring them to the material elements of 

 which organisms are composed, but that something not really 

 belonging to the natural order [ ?] ... is present in living 

 things" (italics mine). 



To me it is entirely obscure why the term "non-perceptual fac- 

 tor," employed by Jennings in a logical and consistent manner, 

 is by him rejected as nonsense. 



His formulation of the non-perceptual is very clear. 



Conditions subject to diverse physical tests will here be called either 

 perceptual or physical. 6 



A non-perceptual agent would be one which though producing at a 

 particular time a particular physical event, was not subject to other 

 physical tests for its presence. 7 



I have given a formulation much resembling this, of what I call 

 the "immaterial factor" in my paper bearing that title, printed 

 in the "Festschrift fur Schwalbe." 



This work was to have been published the first of August, 

 1914, at the very moment of the outbreak of the war. Whether 

 it has been issued I do not know. 



My definition is as follows : 



Als materiell gilt uns im allgemeinen ein Objekt unserer Erkenntniss 

 welches eine Mehrzahl von einandor uualilianin-vr F.iiroiischaften (sc. 

 Wirkungsweisen) in sich vereinigt und sowohl in Tiitigkeit als in Ruhe 

 befindlich wenigstens gedacht werden kann. 



e Johns Hopkins University Circular, 1914, No. 10, p. 8. 



* Ibid., p. 12. 



