METHOD OF DISCOVERING CAUSAL LAWS 183 



abstract,&quot; or rather, &quot;the only one that the concrete facts of ex 

 perience warrant us in regarding as any way plausible or deserving 

 of serious consideration &quot; ? And the latter alternative is, of course, 

 the true one. So, too, the practical question now arises : Over 

 what extent of field must we conduct our negative experiments, 

 or how many must we perform, or when should we be satisfied 

 that our supposed cause is really the only possible, or indispens 

 able, cause ? And the answer will be : when we have allayed 

 all reasonable fears that there may be other causes of the pheno 

 menon ; when we have examined and disproved all the alter 

 natives that we think really worthy of consideration. The decision 

 of this will obviously depend on the nature of the investigation, 

 and on the knowledge, insight, and prudence of the investigator. 



An instructive illustration of the value of the joint method is that furnished 

 by investigations into the cause of fermentation. 1 &quot;When sugar is changed 

 into alcohol and carbonic acid in the ordinary alcoholic fermentation, the pro 

 cess is in some way related to the cells of the yeast plant. . . . For many 

 years these minute organisms received little or no attention ; but in 1838 

 Schwann, one of the founders of the cell theory, and Cagniard de la Tour, 

 demonstrated the vegetable nature of these yeast cells, and showed that they 

 grew and multiplied in saccharine solutions.&quot; The method of single agree 

 ment warranted the conclusion that they were probably a cause of fermenta 

 tion ; but not the conclusion that they were indispensable to fermentation. 

 Liebig contended that they merely formed &quot; a substance which by purely 

 chemical action produces the chemical change called fermentation &quot; a sub 

 stance which might conceivably be produced otherwise than by the action of 

 living germs like the yeast cell. The hypothesis that such living germs cause 

 fermentation not in this indirect way, but by such direct and immediate action 

 that their presence is indispensable and has no possible substitute this hypo 

 thesis had now to be tested. In other words, it had to be proved not only that 

 living germs cause fermentation, but also that nothing else can. Symbolizing 

 cause and effect by G and /% respectively, the two propositions &quot; If G then f,&quot; 

 and &quot; If not G then not /%&quot; had to be established. The first of these pro 

 positions offered no difficulty : it was accepted as sufficiently established for 

 the time by the method of single agreement. At all events, the onus of proving 

 that the presence of living germs in fermentable materials need not necessarily 

 cause fermentation, would have been rightly placed on the shoulders of any 

 one who would venture to put forward such a contention (cf. 207). But to 

 establish the second proposition a careful series of negative experiments had 

 to be conducted : the real difficulty in the case being to get negative instances 

 in which the complete absence of living germs would be assured beyond all 

 reasonable doubt. The following were some of the experiments : (i) &quot; Gay- 

 Lussac showed that clean grapes or boiled grape juice, passed into the 

 Torricellian vacuum of a barometer-tube, remained free from fermentation for 



1 Cf. MELLONE, op. cit., pp. 298-9, 310-11. 



