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SCIENCE 



[N. S. Vol. XXXVIII. No. 9S2 



in the light of modem logical mathematical 

 analysis, to be, I say, simply a leading idea, 

 — that is, a principle which we can neither 

 confirm nor refute by any experience now 

 within our range, but which we use and 

 need in geometry precisely because it is so 

 serviceable in simplifying the geometry of 

 the plane. 



If I may venture to cite an example from 

 your own science, I should suggest the fol- 

 lowing: That fundamental principle of 

 Virchow's "Cellular Pathology" which 

 asserted the origin of every cell from a cell 

 was, as I already said, a perfectly straight- 

 forward induction, of Peiree's first type, 

 that is, it was a probable assertion of a cer- 

 tain constitution as holding for a whole 

 type of eases — an assertion made simply 

 because this constitution had been observed 

 to hold for a sufficient number of fairly 

 selected samples of the type. But, on the 

 other hand, consider another principle which 

 Virchow asserted already in 1847 or earlier, 

 and which, as I have long been told, has 

 been of the first importance for the whole 

 later development of your science: "We 

 have learned to recognize," says Virchow, 

 "that diseases are not autonomous organ- 

 isms, that they are no entities that have 

 entered into the body, that they are no 

 parasites which take root in the body, but 

 that they merely show us the course of the 

 vital processes under altered conditions" 

 {"das sie nur den Ablauf der Lehenser- 

 scheinungen unter verdnderten Bedingun- 

 gen darstellen") . 



Now of course I have nothing to suggest 

 regarding the objective truth of this asser- 

 tion. But I venture to point out that, logic- 

 ally regarded, it is not an hypothesis to be 

 definitely tested by any observation, but is 

 rather an hypothesis of the type of Euclid's 

 postulate about the parallel lines, that is, 

 it is a leading idea. For, on the one hand, 

 how could Virchow regard this principle as 



one that had been definitely tested, and al- 

 ready confirmed by direct observation and 

 experience at a time when, as in 1847, he 

 was not yet possessed even of his own gen- 

 eral principle of a cellular pathology, and 

 when he regarded the whole science of 

 pathology as in its infancy, and the causa- 

 tion of disease as very largely unknown. 

 On the other hand, what experience could 

 one look for that would definitely refute the 

 principle if it were false? Would the ex- 

 perience of such facts as those of your 

 modern bacteriology refute that principle? 

 No, at least so far as I understand the sense 

 of the principle as Virchow stated it in 

 1847. For when bacteria, or when any of 

 their products or accompaniments came to 

 be recognized either as causing disease, or 

 as affecting the course of disease in any 

 way, it was still open to Virchow to say that 

 the causes thus defined simply constitute 

 these very verdnderte Bedingungen under 

 which the Ablauf der Lebenserscheinungen 

 takes place. In other words, the principle, 

 if understood with sufficient generality, 

 simply asserts that a disease can not occur 

 in an organism without the processes of the 

 disease being themselves alterations of the 

 processes of the organism, and such altera- 

 tions as the altered conditions, whatever 

 they are, determine. Such a principle, so 

 understood, seems tolerably safe from em- 

 pirical refutation. It would remain un- 

 refuted, and empirically irrefutable, so far 

 as I can see, even if the devil caused disease. 

 For the devil would then simply be one of 

 the verdnderte Bedingungen. Thus when 

 the devils on a famous occasion entered, in 

 the tale, into the Gaderene swine, the 

 Ablauf of the Lebenserscheinungen of the 

 swine was such, under the verdnderte Bedin- 

 gungen, that, as we are told, they ran down 

 a steep place into the sea. But I do not 

 see that this just stated pathological postu- 

 late of Virchow's need have suffered ship- 



