EVIDENCE OF UNIVERSAL CAUSATION. 397 



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 anal- 

 ogy, 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 observa- 

 tion and experiment, as in the speculations already alluded to respecting 

 the moon and planets, such slight probabilities 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 ex- 

 periment, or affords a reason for trying one experiment rather than anoth- 

 er, may be of the greatest benefit to science. 



On this ground, though I can not accept as positive truths any of those 

 scientific hypotheses which are unsusceptible 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 hypoth- 

 esis both explains known facts, and has led to the prediction of others 

 previously unknown, and since verified by experience, the laws of the phe- 

 nomenon which is the subject of inquiry must bear at least a great similar- 

 ity to those of the class of phenomena to which the hypothesis assimi- 

 lates it ; and since the analogy which extends so far may probably extend 

 further, nothing is more likely to suggest experiments tending to throw 

 light upon the real properties of the phenomenon, than the following 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 illu- 

 sion 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 phenomena between the laws of which and 

 those of the given phenomenon any analogy exists, and trying all such ex- 

 periments as may tend to the discovery of ulterior analogies pointing in 

 the same direction. 



CHAPTER XXI. 



OF THE EVIDENCE OF THE LAW OF UNIVERSAL CAUSATION. 



§ 1. 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 se- 

 quence, are ascertained or tested. As we recognized in the commence- 

 ment, and have been enabled to see more clearly in the progress of the in- 

 vestigation, the basis of all these logical operations is the law of causation. 



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

 edition. 



