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. lint, secondly, it was necessary for the purpose of the calculation to make some assumption concerning the supposed planet. It was, accordingly, 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 remarkable 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 hypothesis 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 purpose 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 working 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 uniformity 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 conclude that there was some kind of uniformity in the configuration and the relations of the elements of the system, he would find himself deceived. Speculations concerning such uniformity are nevertheless 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 conclusions, 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 apparitions 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-scientific times, and it has much to say for itself. Nevertheless, it is scarcely philosophical, and is in no wise demanded by the requirements of science and the conditions of scientific progress. A man may be perfectly 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
