396 INDUCTION. 



earth in any of its derivative properties, such as that of having inhabitants ; 

 though when we consider how immeasurably multitudinous are those of 

 their properties which we are entirely ignorant of, compared with the few 

 which w^e know, we can attach but trifling weight to any considerations of 

 resemblance in which the known elements bear so inconsiderable a propor- 

 tion 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 Grecian, and 

 some to those of Etruscan, or Egyptian art. We are of course supposing 

 that it does not possess any quaUty which has been ascertained, by a suffi- 

 cient induction, to be a conclusive mark either of the one or of the other. 



/ § 3. Since the value of an analogical argument inferring one resemblance 

 /from other resemblances without any antecedent evidence of a connection 

 / between them, depends on the extent of ascertained resemblance, compared 

 I first with the amount of ascertained difference, and next with the extent of 

 I the unexplored region of unascertained properties; it follows that where 

 \ the resemblance is very great, the ascertained difference very small, and our 

 jknowledge 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 known 

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

 in some few, we may reasonably expect to find in the unobserved remain- 

 der of its properties, a general agreement with those of the former ; but 

 also a difference corresponding 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 known, 

 when consequently the observed order of their occurrence amounts only to 

 an empirical law, it often happens that the conditions which have co-exist- 

 ed 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 

 inference that the effect will occur, notwithstanding this deficiency of com- 

 plete resemblance to the cases in which it has been observed, may, though 

 of the nature of analogy, possess a high degree of probability. It is hard- 

 ly necessary to add that, however considerable this probability may be, no 

 competent inquirer into nature will rest satisfied with it when a complete 

 induction is attainable; but will consider the analogy as a mere guide- 

 post, pointing out the direction in which more rigorous investigations 

 should b^ pi'osecuted. 

 It is in tfiis last respect that considerations of analogy have the highest 



