ANALOGY. 395 



sembles the earth in being a solid, opaque, nearly spherical substance, ap- 

 pearing to contain, or to have contained, active volcanoes ; receiving heat 

 and light from the sun, in about the same quantity as our earth ; revolving 

 on its axis ; composed of materials which gravitate, and obeying all the va- 

 rious 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 inhabit- 

 ants in that luminary would derive from these various resemblances to 

 the earth, a greater degree of probability than it would otherwise have ; 

 though the amount of the augmentation it would be useless to attempt to 

 estimate. 



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

 not known to be immaterial with respect to m, forms some additional rea- 

 son for presuming that B has the attribute my it is clear, ^ contra, that ev- 

 ery 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, pro- 

 duce 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 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 may be deemed to prepon- 

 derate, the prob:iLility derived from analogy will be for or against B's hav- 

 ing 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 Hght, no 

 clouds, and (it is therefore concluded) no water. These differences, consid- 

 ered merely as such, might perhaps balance the resemblances, so that anal- 

 ogy 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 consequence, 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 favor of, the moon's being inhabited. Since life can not 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 which and the 

 earth there is a much closer resemblance; which possess an atmosphere, 

 clouds, consequently water (or some fluid analogous to it), and even give 

 strong indications of snow in their polar regions ; while the cold, or heat, 

 though differing greatly on the average from ours, is, in some parts at least 

 of those planets, possibly not more extreme than in some regions of our 

 OAvn which are habitable. To balance these agreements, the ascertained 

 differences are chiefly in the average light and heat, velocity of rotation, 

 density of material, intensity of gi-avity, and similar circumstances of a sec- 

 ondary kind. With regard to these planets, therefore, the argument of 

 analogy gives a decided preponderance in favor of their resembling the 



