YALUE OF INDIRECT INFERENCE. 355 



bination and permutation of ideas, and inference, all the 

 possible causes and explanations. Next, by similar means, 

 we make a list of all the possible effects of excluding one 

 of the conditions ; and having, by means of experiment, 

 excluded that condition or circumstance, and thus gained 

 additional knowledge of facts, we repeat the reasoning 

 process, and exclude another condition by experiment ; 

 and so on, until we have disproved and mentally excluded 

 every explanation, except the correct one. As the several 

 hypotheses which we raise at each step of the process 

 possess individually very different degrees of probability, 

 and as the several conditions to be tested by exclusion 

 possess different degrees of importance, we usually ex- 

 amine the most likely and important ones first, and. we 

 determine their relative degrees of likelihood and im- 

 portance not usually by a strict calculation of probabilities, 

 but by a crude and ready process of guessing. Usually 

 also we do not test every possible hypothesis, because some 

 have so small a degree of probability that we may safely 

 disregard them. From the collection of results thus ob- 

 tained we draw a number of logical conclusions which 

 collectively contain the essential characters of the phe- 

 nomenon we have investigated, and we thus unfold and 

 render explicit some of the chief truths it contains. 1 



The following is a well-described example of the 

 value of logic in scientific discovery : 'In Sir Humphry 

 Davy's experiments upon the decomposition of water by 

 galvanism, it was found that besides the two components 

 of water, oxygen and hydrogen, an acid and an alkali were 

 developed at the two opposite poles of the machine. As 

 the theory of the analysis of water did not give reason to 



1 This process agrees with the description of that of indirect in- 

 ference given in Jevons's Principles of Science, vol. i. 2nd edit. p. 89. 



A A 2 



