192 INDUCTION. 



their truth to be conthigent upon some other circumstance not origi- 

 nally attended to. It will appear, I think, from the subsequent part of 

 our inquiry, that there is no logical fallacy in this mode of proceeding ; 

 but we may see already that any other mode is rigorously impractica- 

 ble : since it is impossible to frame any scientific method of induction, 

 or test of the correctness of inductions, vmless ujjon the hypothesis that 

 some inductions of unquestionable certainty have been already made. 



Let us revert, for instance, to one of our foraier illustrations, and 

 consider why it is that, with exactly the same amount of evidence, 

 both negative and positive, we did not reject tlie as.sertion that there 

 are black swans, while we should refuse credence to any testimony 

 which assexted that there were men weai'ing their heads underneath 

 their shoulders. The first assertion was more ci'edible than the latter. 

 But why more credible ? So long as neither phenomenon had been 

 actually witnessed, what reason was there for finding the one harder 

 to be believed than the other 1 Apparently, because there is less con- 

 stancy in the colors of animals, than in the general structure of their 

 internal anatomy. But how do we know this ] Doubtless, fi-om 

 experience. It appears, then, that we need experience to inform us, 

 in what cases, or in what sorts of cases, experience is to be relied 

 upon. Experience must be consulted in order to learn from it under 

 what circumstances arguments from it will be valid. We have no 

 ulterior test to which we subject expei'ience in general ; but we make 

 experience its own test. Experience testifies, that among the uni- 

 formities which it exhibits or seems to exhibit, some are more to be 

 relied upon than others ; and uniformity, therefore, may be presumed, 

 from any given number of instances, with a gi'eater degree of assurance, 

 in proportion as the case belongs to a class in which the nniformities 

 have hitherto been found more uniform. 



•This mode of coixectihg one generalization by means of another, 

 a narrower generalization by a wider, which common sense suggests 

 and adopts in practice, is the i-eal type of scientific Induction. All 

 that art can do is but to give accuracy and precision to this process, 

 and adapt it to all varieties of cases, without, any essential alteration in 

 its principle. 



There are of course no means of ai:)plying such a test as that above 

 described, unless we already possess a general knowledge of tlie 

 prevalent character of the uniformities existing throughout natiu'e. 

 The indispensable foundation, therefore, of a scientific formula of 

 induction, must be a survey of the inductions to which mankind have 

 been conducted in unscientific practice ; with the special purpose of 

 ascertaining -what kinds of uniformities have been found perfectly 

 invariable, pei-vading all nature, and what ai'e those which have been 

 found to vary with difference of time, place, or other changeable 

 circumstances. 



§ 3. The necessity of such a survey is confirmed by the considera- 

 tion, that the stronger inductions are the touchstone to which we 

 always endeavor to bring the weaker. If we find any means of 

 deducing one of the less strong inductions from stronger ones, it 

 acquires, at once, all the strength of those from which it is deduced ; 

 and even adds to that strength ; since the independent experience on 

 which the weaker induction previously rested, becomes additional 



