ANALOGY. 



367 



polar regions ; while the cold or heat, 

 though differing greatly on the aver- 

 age from ours, is, in some parts at 

 least of those planets, possibly not 

 more extreme than in some regions 

 of our own which are habitable. To 

 balance these agreements, the ascer- 

 tained differences are chiefly in the 

 average light and heat, velocity of 

 rotation, density of material, inten- 

 sity of gravity, and similar circum- 

 stances of a secondary kind. With 

 regard to these planets, therefore, the 

 argument of analogy gives a decided 

 preponderance in favour of their re- 

 sembling the earth in any of its de- 

 rivative properties, such as that of 

 having inhabitants ; though, when we 

 consider how immeasurably multitu- 

 dinous are those of their properties 

 which we are entirely ignorant of, 

 compared with the few which we 

 know, we can attach but trifling 

 weight to any considerations of re- 

 semblance in which the known ele- 

 ments bear so inconsiderable a pro- 

 portion to the unknown. 



Besides the competition between 

 analogy and diversity, there may be 

 a competition of conflicting analogies. 

 The new case may be similar 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 products. A painting of un- 

 known origin may resemble, in certain 

 of its characters, known works of a 

 particular master, but in others it 

 may as strikingly resemble those of 

 some other painter. A vase may 

 bear some analogy to works of Gre- 

 cian, and some to those of Etruscan 

 or Egyptian art. We are of course 

 supposing that it does not possess 

 any quality which has been ascer- 

 tained, by a sufficient induction, to 

 be a conclusive mark either of the 

 one or of the other. 



§ 3. Since the value of an ana- 

 logical argument inferring one re- 

 semblance from other resemblances 



without any antecedent evidence of 

 a connection between them, depends 

 on the extent of ascertained re- 

 semblance, compared first with the 

 amount of ascertained difference, and 

 next with the extent of the unex- 

 plored region of unascertained pro- 

 perties ; it follows that where the 

 resemblance is very great, the ascer- 

 tained difference very small, and our 

 knowledge of the subject-matter toler- 

 ably 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 dis- 

 cover, for example, an unknown ani- 

 mal or plant, resembling closely some 

 known one in the greater number of 

 the properties we observe in it, but 

 differing in some few, we may reason- 

 ably expect to find in the unobserved 

 remainder of its properties a gene- 

 ral agreement with those of the for- 

 mer, but also a difference correspond- 

 ing proportionately to the amount of 

 observed diversity. 



It thus appears that the conclusions 

 derived from analogy are only of any 

 considerable value when the case to 

 which we reason is an adjacent case ; 

 adjacent, not as before, in place or 

 time, but in circumstances. In the 

 case of effects of which the causes are 

 imperfectly or not at all knowTi, when 

 consequently the observed order of 

 their occurrence amounts only to an 

 empirical law, it often happens that 

 the conditions which have co-existed 

 whenever the effect was observed 

 have been very numerous. Now if a 

 new case presents itself, in which all 

 these conditions do not exist, but the 

 far greater part of them do, some one 

 or a few only being wanting, the in- 

 ference that the effect will occur, not- 

 withstanding this deficiency of com- 

 plete resemblance to the cases in 

 which it hai been observed, may, 

 though of the nature of analogy, poa^ 



