ch. in.] THE TEST OF TRUTH 65 



a single experiment, that nitrogen will not support combus- 

 tion, we believe that it will be just the same through all 

 future time. If we withhold our assent, " it is from a doubt 

 whether the one experiment was properly made, not whether 

 if properly made it would be conclusive." * Here, then, as 

 Mr. Mill says, " is a general law of nature inferred without 

 hesitation from a single instance ; a universal proposition 

 from a singular one. Now mark another case, and contrast 

 it with this. Not all the instances which have been observed 

 since the beginning of the world, in support of the general pro- 

 position that all crows are black, would be deemed a sufficient 

 presumption of the truth of the proposition, to outweigh the 

 testimony of one unexceptionable witness who should affirm 

 that in some region of the earth not fully explored, he had 

 caught and examined a crow, and had found it to be grey." 



What is the explanation of this difference ? " Why is a 

 single instance in some cases sufficient for a complete induc- 

 tion, while in others myriads of concurring instances, without 

 a single exception known or presumed, go such a very little 

 way towards establishing a universal proposition ? " The 

 solution is to be sought in the extreme complexity of the 

 conditions in the one case as contrasted with their extreme 

 simplicity in the other. The scientific thinker does not con- 

 sider blackness a necessary attribute of a crow, because he 

 believes that some inappreciable variation in the nutrition of 

 the bird, by altering the deposit of pigment in the feathers, 

 might give us a grey or a white crow instead of a black one. 

 Or if we do not reflect upon the matter so carefully as this, 

 we at least regard a crow as a very complex aggregate of con- 

 ditions and results, and find no difficulty in imagining that 

 some of the conditions varying might affect the sum-total of 

 results. Or if this also be taken to imply too much conscious 

 rhilosophizing in us, it is undeniable that our conception of a 

 cro , as of any other vertebrate, is made up of a large number 

 1 Mill, System of Logic, vol i. p. 352. 



