334 INDUCTION. 



For example, I might infer that there are probably inhabitants in 

 the moon, because there are inhabitants on the earth, in the sea, and in 

 the air ; and this is the evidence of analogy. The circumstance of 

 having inhabitants is here assumed not to be an ultimate property, but 

 (as it is reasonable to suppose) a consequence of other properties ; 

 and depending, therefore, in the case of our earth, upon some of its 

 properties as a portion of the universe, but upon which of those prop- 

 erties we. know not. Now, the moon resembles the earth in being a 

 solid, opaque, nearly spherical substance ; containing active volcanoes ; 

 receiving heat and light from the sun, in about the same quantity as 

 our earth ; revolving on its axis ; whose materials gravitate, and which 

 obey all the various laws resulting from that property. And I think 

 no one will deny that if this were all that was known of the moon, the 

 existence of inhabitants in that luminary would derive from these 

 various resemblances to the earth, a greater degree of probability 

 than it would otherwise have : although the amount of the augmenta- 

 tion it would be ridiculous to attempt to estimate. 



If, however, every resemblance proved between B and A, in any 

 point riot known to be immaterial with respect to m, forais some addi- 

 tional reason for presuming that B has the attribute in ; it is clear e con- 

 tra, that every dissimilarity which can be proved between them, fur- 

 nishes 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 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 proper- 

 ties, and that the differences which are unknown will on the average 

 of cases bear some proportion to those which are known. There will, 

 therefore, 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 \n\\ be for or against B's having the property in. The 

 moon, for instance, agrees with the earth in the circumstances ah'eady 

 mentioned; but differs in being smaller, in having its surface more 

 unequal, and apparently 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 presumption either way. But considering that some of the circum- 

 stances which are wanting on the moon are among those which, on our 

 earth, are found to be indispensable conditions of aniinal life, we may 

 conclude 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 diffel-ences fi-ora the 

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

 resemblances which exist become presumptions against, not in favor of, 



in combustible substances ; and on similar grounds he conjectured that water, though not 

 combustible, contained a combustible ingredient. Experiment having subsequently shown 

 that in both instances he guessed right, the prophecy is considered to have done great 

 honor to his scientific sagacity; but it is to this day uncertain whether the praise was 

 merited ; whether the guess was, in truth, what there are so many examples of in the 

 history of science, a far-sighted anticipation of a law afterwards to be discovered. The 

 progress of science has not hitherto shown ground for believing that there is any real con- 

 nexion between combustibility and a high refracting power. 



