368 



INDUCTION. 



sesB a high degree of pi-obability. 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 com- 

 plete 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 con- 

 siderations of analogy have 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 ana- 

 logy, however faint, which may not be 

 of the utmost value in suggesting ex- 

 periments or observations that may 

 lead to more positive conclusions. 

 When the agents and their effects are 

 out of the reach of further observa- 

 tion and experiment, as in the specu- 

 lations already alluded to respecting 

 the moon and planets, such slight pro- 

 babilities are no more than an inter- 

 esting theme for the pleasant exercise 

 of imagination ; but any suspicion, 

 however slight, that sets an ingenious 

 person at work to contrive an experi- 

 ment, or affords a reason for trying 

 one experiment rather than another, 

 may be of the greatest benefit to 

 science. 



On this ground, though I cannot 

 accept as positive truths any of those 

 scientific hypotheses which are unsus- 

 ceptible 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 diame- 

 trically 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 hypo- 

 thesis both explains known facts and 

 has led to the prediction of others 

 previously unknown, and since veri- 

 fied 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 phe- 

 nomena to which the hypothesis as- 

 similates it ; and since the analogy 

 which extends so far may probably 

 extend farther, nothing is more likely 

 to suggest experiments tending to 

 throw light upon the real properties 

 of the phenomenon than the follow- 

 ing out such an hypothesis. But to 

 this end it is by no means necessary 

 that the hypothesis be mistaken for a 

 scientific truth. On the contrary, that 

 illusion is in this respect, as in every 

 other, an impediment to the progress 

 of real knowledge, by leading inquirers 

 to restrict themselves arbitrarily to 

 the particular hypothesis which is 

 most accredited at the time, instead of 

 looking out for every class of pheno- 

 mena between the laws of which and 

 those of the given phenomena any 

 analogy exists, and trying all such 

 experiments as may tend to the dis- 

 covery of ulterior analogies pointing 

 in the same direction. 



CHAPTER XXL 



OF THE EVIDENCE OF THE LAW OP 

 UNIVERSAL CAUSATION. 



§ I. We have now completed our 

 review of the logical processes by 

 which the laws, or uniformities, of 

 the sequence of phenomena, and those 

 uniformities in their co-existence 

 which depend on the laws of their 

 sequence, are ascertained or tested. 

 As we recognised in the commence- 

 ment, and have been enabled to see 



* Hartley's Observations on Man, vol. i, 

 p. 1 6. The passage is not in Priestley's 

 curtailed edition. 



