THE UNIFORMITY OF NATURE. Z57 



reference to the discovery of the planet Neptune. Two working 

 hypotheses were necessary in this case. First, there was the great 

 hypothesis of gravitation according to the Newtonian law. But, 

 secondly, it was necessary for the purpose of the calculation to make 

 some assumption concerning the supposed planet. It was, accord- 

 ingly, assumed that Bode's empirical law of planetary distances was 

 true, and that, if the planet existed, its distance would be given by 

 this law. The position of the planet was determined by the remark- 

 able calculations of Adams and Leverrier ; and what was the result ? 

 That the first hypothesis was confirmed, if it needed confirmation, 

 and that the second was exploded, when the distance of Neptune 

 came to be determined by actual observation. Thus a working hy- 

 pothesis was proved to be false ; but no harm was done. Neptune was 

 discovered, though his distance had been wrongly assumed ; the working 

 hypothesis had fortunately been near enough to the truth for the pur- 

 pose in hand, and, having served that purpose, it could be flung away. 



But in speaking of a working hypothesis it should be carefully 

 borne in mind that the very epithet icorJcing indicates limits within 

 which the work must take place. The hypothesis of the uniformity 

 of Nature, being founded upon or suggested by the discovery of uni- 

 formity in a certain department, must be carefully confined to similar 

 departments, or, at all events, must be regarded with suspicion if it 

 goes beyond them. "We have already seen that if an astronomer, from 

 the uniformity of mechanical action in the solar system, should con- 

 clude that there was some kind of uniformity in the configuration 

 and the relations of the elements of the system, he would find him- 

 self deceived. Speculations concerning such uniformity are neverthe- 

 less very tempting. Kepler, as will be remembered, could not resist 

 them, and got into some quagmires in consequence. But the temptation 

 must be resisted ; an assumed uniformity may lead to serious errors, if 

 it goes beyond the strictly physical region to which it belongs. 



And this view of the matter leads, as it seems to me, to sound con- 

 clusions, with regard to the relation in which the truth of the uniformity 

 of Nature stands to truths, or supposed truths, of a different kind. 



Take, for example, the case of alleged apparitions. I imagine that 

 the tendency in the minds of not a few among us is to ignore appa- 

 ritions utterly and completely. They are supernatural, and that is 

 enough ; they do not conform themselves to the recognized laws of 

 mechanics, optics, acoustics, motion. This is a rebound from the old 

 facility in accepting tales of demonology and witchcraft in pre-scien- 

 tific times, and it has much to say for itself. Nevertheless, it is scarcely 

 philosophical, and is in no wise demanded by the requirements of sci- 

 ence and the conditions of scientific progress. A man may be per- 

 fectly orthodox in his physical creed, and yet may admit the weight 

 of evidence in favor of certain alleged phenomena which will not square 

 themselves with physics. Such alleged phenomena are not necessarily 

 VOL. XX vm. — 17 



