ANALOGY. 101 



tra, that every dissimilarity which can be proved 

 between them, furnishes a counter-probability of the 

 same nature on the other side. It is not indeed 

 impossible that different ultimate properties may, in 

 some particular instances,, produce the same deriva- 

 tive property ; but on the whole it is certain that 

 things which differ in their ultimate properties, will 

 differ at least as much in the aggregate of their deri- 

 vative properties, and 'that the differences which are 

 unknown will en the average of cases bear some pro- 

 portion to those %hich' are kilo wii. : There will, there- 

 fore, be a competition between the known points of 

 agreement and the known points of difference in A and 

 B ; and according as the one or the other are deemed 

 to preponderate, the probability derived from analogy 

 will be for or against B's having the property m. The 

 moon, for instance, agrees with the earth in the cir- 

 cumstances already mentioned; but differs in being 

 smaller, in having its surface more unequal, and appa- 

 rently volcanic throughout, in having no atmosphere 

 sufficient to refract light, no clouds, and therefore (it is 

 inferentially concluded) no water. These differences, 

 considered merely as such, might perhaps balance 

 the resemblances, so that analogy would afford no pre- 

 sumption either way. But considering that some of 

 the circumstances which are wanting on the moon are 

 among those which, on our earth, are found to be 

 indispensable conditions of animal life, we may con- 

 clude that if that phenomenon does exist in the moon, 

 it must be as the effect of causes totally different from 

 those on which it depends here ; as a consequence, 

 therefore, of the moon's differences from the earth, 

 not of their points of agreement. Viewed in this 

 light, all the resemblances which exist become pre- 

 sumptions against, not in favour of, her being inha- 



