92 INDUCTION. 



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 proba- 

 bility 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 remainder 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 conse- 

 quently 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 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 

 complete 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 hardly necessary to add that, how- 

 ever 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 be prosecuted. 



It is in this last respect that considerations of analogy have 



