HYPOTHESIS 157 



inferred from them, just in so far as they are likely to be causally 

 connected with this latter. The points of difference, too, must be 

 noted; and these will be &quot;important&quot; as militating against the 

 analogy, and so weakening it, in so far as they appear to be of 

 such a kind as would be incompatible with the supposed causal 

 connexion. This supposition of a causal connexion has next to 

 be tested by observation and, if possible, experiment according 

 to methods set forth in the next chapter. Should these convince 

 us that the resemblances were merely accidental in regard to the 

 inferred characteristics, that the latter cannot really be affirmed 

 of the phenomenon under investigation, then the analogy of the 

 latter to the other phenomenon was bad and misleading from the 

 start. If, on the other hand, further analysis reveals some sort of 

 causal connexion which enables us either to verify our hypothesis, 

 or to modify it in some way, to alter its scope and restate it, and 

 to verify it in its altered condition, then the analogy will have 

 been so far good and useful and instructive. 



We have referred to analogy as the application of known laws 

 to new sets of facts. Such attempts at extension usually lead 

 to restatements which give such laws a wider scope ; or to the 

 suggestion and verification of new hypotheses. These are the most 

 important functions of analogy in induction. We may illustrate 

 them by the following example, for which we are indebted to Dr. 

 Mellone s Text-book: 



&quot; A conspicuous instance ... is seen in the early researches of Pasteur and 

 his friends into bacteriology, as described in the Life of Louis Pasteur by his 

 son-in-law. The old belief was that many contagious diseases were due to a 

 virus or poison introduced into the blood. Further research was undertaken 

 on the assumption that the cause of the diseases was something in the blood, 

 but not necessarily a virus. This was a suggestion by analogy with the 

 former belief, and it was experimentally proved by inoculating healthy 

 animals with a drop of the infected blood. Afterwards the presence of minute 

 animalculae, visible only by the microscope, was detected in the blood of 

 diseased animals ; but at first it was supposed that these minute organisms 

 could not produce such great effects. But subsequently Pasteur proved that 

 such a great effect as fermentation was caused by the growth of an invisible 

 vegetable organism ; hence analogy suggested that the animalculae whose 

 presence was detected in the infected blood, might after all be the true cause 

 of the diseases in question. This hypothesis, being experimentally verified, 

 was proved to be true by applications of the joint method \cf. 242]. The old 

 theory, that these diseases were caused by a virus introduced into the blood, 

 could only give a forced explanation of many known facts ; and it had to give 

 way to a new theory harmonising all the facts. But the new theory was 

 originally suggested by analogy with the old ; and the speculations with 



