ANALOGY. 93 



the highest scientific value. The cases in which analogical 

 evidence affords in itself any very high degree of probability, 

 are, as we have 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 further observation and experiment, as in the specu- 

 lations 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 work to con- 

 trive an experiment, or affords a reason for trying one experi- 

 ment rather than another, may be of the greatest benefit to 

 science. 



On this ground, though I cannot accept as positive doc- 

 trines any of those scientific hypotheses which are unsuscep- 

 tible of being ultimately brought to the test of actual induction, 

 such, for instance, as the two theories of light, the emission 

 theory of the last century, and the undulatory theory which 

 predominates in the present, I am yet unable to agree with 

 those who consider such hypotheses to be worthy of entire 

 disregard. As is well said by Hartley (and concurred in by 

 a thinker in general so diametrically opposed to Hartley's 

 opinions as Dugald Stewart), " any hypothesis which 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 experimenta crucis for the sake of 

 future inquirers."* If an hypothesis both explains known 

 facts, and 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 may probably extend farther, nothing is more 



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

 Priestley's curtailed edition. 



