90 INDUCTION. 



forms some additional reason for presuming that B has the 

 attribute m ; it is clear, e contra, 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 unusual 

 that different ultimate properties should, in some particular 

 instances, produce the same derivative 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 

 derivative properties, and that the differences which are un 

 known will on the average of cases bear some proportion to 

 those which are known. There will, therefore, be a competi 

 tion between the known points of agreement and the known 

 points of difference in A and B ; and according as the one or 

 the other may be 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 circumstances already mentioned ; but differs in being 

 smaller, in having its surface more unequal, and apparently 

 volcanic throughout, in having, at least on the side next the 

 earth, no atmosphere sufficient to refract light, no clouds, and 

 (it is therefore concluded) no water. These differences, con 

 sidered merely as such, might perhaps balance the resemblances, 

 so that analogy would afford no presumption either way. But 

 considering that some of the circumstances which are wanting 

 on the moon are among those which, on the earth, are found to 

 be indispensable conditions of animal life, we may conclude that 

 if that phenomenon does exist in the moon, (or at all events 

 on the nearer side,) it must be as an effect of causes totally 

 different from those on which it depends here; as a conse 

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

 not of the points of agreement. Viewed in this light, all the 

 resemblances which exist become presumptions against, not in 

 favour of, the moon s being inhabited. Since life cannot exist 

 there in the manner in which it exists here, the greater 

 the resemblance of the lunar world to the terrestrial in other 

 respects, the less reason we have to believe that it can contain 

 life. 



There are, however, other bodies in our system, between 



