336 INDUCTION. 



It thus appears that the conclusions derived from analogy are only 

 ■of any considerable value, w^hen the case to which we reason is an ad- 

 jacent case; adjacent, not as before, in place or time, but in circum- 

 stances. In the case of effects of which the causes are impeiiectly or 

 not at all known, when consequently the observed order of their oc- 

 cuiTcnce 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 wall 

 occur notwithstanding this deficiei>cy of complete resemblance to the 

 cases in which it has been observed, may, although of the nature of 

 analogy, possess a high degi-ee of probability. It is hardly necessary 

 to add that, however considerable this probability may be, no com- 

 petent 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 direction in which more rigorous 

 investigations should be prosecuted. 



It is in this last respect that considerations of analogy have the high- 

 est 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 suggesting experiments or observations that 

 may lead to more positive conclusions. When the agents and their 

 effects are out of the reach of fiirther observation and experiment, as 

 in the speculations already alluded to respecting the moon and planets, 

 such slight probabilities are no more than an interesting theme for the 

 pleasant exercise of imagination ; but any suspicion, however slight, 

 that sets an ingenious person at Avork to contrive an experiment, or 

 that affords a reason for trying one experiment rather than another, 

 may be of eminent service to philosophy. 



On this gi-ound, notwithstanding the unfavorable judgment which I 

 have concurred with M. Comte in passing upon those scientific hypo- 

 theses (when considered as positive doctrines) which are unsusceptible 

 of being ultimately brought to the test of actual induction, such for in- 

 stance as the two theories of light, the emission theory of the last cen- 

 tury, and the undulatory theory which predominates in the present ; I 

 am yet unable to agree with M. Comte in considering those hypo- 

 theses to be worthy of entire disregard. As is well said by Hartley 

 (and concurred in by a philosopher in general so diameti-ically opposed 

 to Hartley's views as Dugald Stewart), " any hypothesis that has so 

 much plausibility as to explain a considerable number of facts, helps 

 us to digest these facts in proper order, to bring new ones to light, 

 and make experivienta crucis for the sake of future inquirers."* If an 

 hypothesis not only explains known facts, but has led to the prediction 

 of others previously unknown, and since verified by experience, the 

 laws of the phenomenon which is the subject of inquiry must bear at 

 least a great similarity to those of the class of phenomena to which the 

 hypothesis assimilates it; and since the analogy which extends so far 



* Hartley'.s Observations on Man, vol. i., p. 16. The passage is not in Priestley's cur- 

 tailed edition. 



