ANALOGY. 335 



her 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 all other respects, the less reason we have to bolievo 

 tliat it can contain life. 



There ai'e, however, other bodies in our system, between which and 

 the earth there is a much closer resemblance; which possess an atmos- 

 phere, 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 avcu'age from ours, is, in 

 some parts at least of those planets, jjossibly not more extreme than in 

 some regions of our own wliich are habitubli;. To balance these agree-; 

 ments, the ascertained differences are chiefly in the average light and 

 heat, velocity of rotation, intensity of gravity, and similar circumstances 

 of a secondary kind. With regard to these planets, therefore, the argu- 

 ment of analogy gives a decided preponderance in favor of their resem- 

 bling the earth in any of its derivative properties, such as that of having 

 inliabitants : though, when we consider how immeasurably multitudi- 

 nous are those of their properties which we are entirely ignorant of, 

 compared with the few wliich we know, we cannot attach more than 

 a very trifling weight to any considerations of resemblance in which 

 the known elements bear so inconsiderable a proportion to the un- 

 known. 



Besides the competition between analogy and diversity, there may 

 be a competition of conflicting analogies. The new case may be sim- 

 ilar in some of its circumstances to cases in which the fact m exists, 

 but in others to cases in which it is known not to exist. Amber has 

 some properties in common with vegetable, others with mineral pro- 

 ducts. A painting, of unknown origin, may resemble, in certain of its 

 characters, known works of a particular master, but in others it may^ 

 as strikingly resemble productions known not to be his. A vase may 

 bear some analogy to works of Grecian, and some to those of Etruscan 

 or Egyptian art. We are of course supposing that it does not possess 

 any quality which has been ascertained, by a sufficient induction, to be 

 a conclusive mark either of the one or of the other. 



§ 3. Since the value of an analogical argument infen-ing one resem- 

 blance from other resemblances without any antecedent evidence of a 

 connexion between them, depends upon the extent of ascertained 

 resemblance, compared first with the amount of ascertained difference, 

 and next with the extent of the unexplored region of unascertained 

 properties; it follows that where the resemblance is very great, the 

 ascertained difference very small, and our knowledge of the subject- 

 matter tolerably extensive, the argument from analogy may approach, 

 in strength very near to a valid induction. If, after much observation 

 of B, we find that it agrees with A in nine out of ten of its known 

 properties, we may conclude with a probability of nine to one, that it 

 will possess any given derivative property of A. If we discover, for. 

 example, an unknown animal or plant, resembling closely some knovm 

 one in the greater number of the properties we observe in it, but dif- 

 fering in some few, we may reasonably expect to find in the unob- 

 served remainder of its properties, a general agreement with those of 

 the former ; but also a difference, corresponding proportionally to the 

 amount of obseiTed diversity. 



