104 INDUCTION. 



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 imper- 

 fectly 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 coexisted whenever the effect was ob- 

 served, 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 

 complete resemblance to the cases in which it has 

 been observed, may, although of the nature of ana- 

 logy, possess a high degree of probability. It is 

 hardly necessary to add that, however considerable 

 this probability may be, no competent inquirer into 

 nature will rest satisfied with it when it is possible to 

 obtain a complete induction; but will consider the 

 analogy as a mere guide-post, pointing out the direc- 

 tion in which more rigorous investigations should be 

 prosecuted. 



It is in this last respect that considerations of 

 analogy have the highest philosophical value. The 

 cases in which analogical evidence affords in itself any 

 very high degree of probability, are, as we have just 

 observed, only those in which the resemblance is very 

 close and extensive; but there is no analogy, however 

 faint, which may not be of the utmost value in sug- 

 gesting experiments or observations that may lead to 

 more positive conclusions. When the agents and 

 their effects are out of the reach of further observation 

 and experiment, as in the speculations already alluded 

 to respecting the moon and planets, such slight pro- 



