ANALOGY. 105 



babilities are no more than an interesting theme for 

 the pleasant exercise of imagination; but any sus- 

 picion, however slight, that sets an ingenious person 

 at work 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 ground, notwithstanding the unfavourable 

 judgment which I have concurred with M. Comte in 

 passing upon those scientific hypotheses (when consi- 

 dered as positive doctrines) which are unsusceptible 

 of being ultimately brought to the test of actual induc- 

 tion, 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 M. Comte in consider- 

 ing those hypotheses to be worthy of entire disregard. 

 As is well said by Hartley (and concurred in by a 

 philosopher in general so diametrically opposed to 

 Hartley's views as Dugald Stewart), u any hypo- 

 thesis 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 experimenta 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 may probably extend further, nothing is more 

 likely to suggest experiments tending to throw light 



* HARTLEY'S Observations on Man, vol. i., p. 16. The passage is 

 not in Priestley's curtailed edition. 



