METHOD OF DISCOVERING CAUSAL LAWS 201 



to the presence of living germs held in suspension in the atmosphere ; that these 

 germs find in fermentible liquids, or in decomposing organic matter, suitable 

 media for multiplication. How are \ve to verify our hypothesis ? 



1. We expose to the air a number of vessls all filled with such liquids, and 

 we find that wherever such germs can have access to them they ferment : the 

 supposed &quot; spontaneous &quot; generation takes place. Here we have the method 

 of agreement. 



2. Next, we secure other vessels similarly filled and take care to seal them 

 hermetically in the purest atmosphere available, upon the Alps, for example, 

 where there is little danger of encountering such germs. We lay the vessels 

 aside, and there they remain indefinitely without any sign of fermentation evi 

 dently free from any living organism. Here we have negative instances which 

 agree in the absence of the supposed cause ; and which, together with the 

 positive instances in (i), illustrate the combination of agreement and difference. 



3. We next expose to the air a variety of similarly filled vessels in a 

 variety of different localities where the atmosphere is tainted in different degrees 

 by the presence of such germs ; for example, in cellars where the air is cold 

 and still and all the germs probably settled, or far up on some high mountain 

 where the air is most likely free from organic impurities, or in the hot, tainted 

 atmosphere of a city where germs of all sorts abound, or in the moist shade of 

 trees or shrubs, in the summer months, when the air is known to swarm with 

 living microbes : we find the rapidity of fermentation, and the amount of living 

 organisms produced in the liquids, to be in proportion to the degree of impurity 

 of the atmosphere in each case. Here we have the Method of Concomitant 

 Variations. From which experiments we infer that life is caused in all cases 

 by the multiplication of some existing living thing, and, negatively, that non 

 living matter cannot give rise to life (cf. 229). 



Living things, therefore, must be endowed with the natural property of 

 reproducing their kind upon the earth. The natural origin of a new living 

 organism is an already existing organism : Omne vivum ab ovo ; omnis cellula 

 ex cellula. 



246. QUANTITATIVE DETERMINATION : MODES OF MEASURE 

 MENT. Very little reflection will show that the progress of dis 

 covery in the physical sciences has been proportionate to the 

 degree in which exact measurement of phenomena has been found 

 to be feasable. * The sciences of physics and chemistry have passed 

 gradually from the study of qualities to the study of quantities : 

 their laws are now not regarded as exact until they are stated 

 quantitatively, or in mathematical formulae. But knowledge of 

 the quantitative aspect of things is not all knowledge of them ; 

 and, moreover, there are vast departments of facts whose quanti 

 tative aspect is of very minor importance to a scientific know 

 ledge of them. Hence, the attempt made in the last century 

 to reduce all the sciences to terms of mechanics (201, 217, 224, 



1 Cf. L. POINCARE, The New Physics (International Scientific Series), chap. ii. ; 

 JOYCE, Principles of Logic, pp. 363 sqq. ; WELTON, op. cit., pp. 160-5, 182-7. 



