560 j. M. KIGG:

pirical verification, and so is the Copernican theory : they explain phenomena, but not by means of causes which are themselves phenomena. If then, it will be asked, laws of causation are hypotheses, and as such liable to supersession, what becomes of the im- mutability which is commonly supposed to be essential to a law of nature? The answer seems to be that the idea which underlies our ascription of immutability to laws of nature does not stand or fall with the truth or otherwise of laws of causation, but is really identical with the uniformity of nature. The uniformity of nature involves five prin- ciples, which are : (1) that every event is the index of a cause or combination of causes ; (2) that no event is the index of more than one cause or combination of causes ; (3) that, given the proper cause or combination of causes, in the absence of counteracting causes, the effect always occurs ; (4) that the sum total of ultimate causes is a fixed quantity ; (5) that the causes existing in the universe are so related, inter se, that in virtue of their mutual modification change, though incessant, is on the whole gradual. The first of these principles we have already discussed at length ; the second is clearly required by the uniformity of nature. If the same effect might indicate any one of several alternative causes, there would manifestly be a breach of uniformity. In the case e.g., of heat, where if anywhere there might seem to be a plurality of causes (as friction, percussion, electrical and chemical action), the uniformity of nature re- quires and science has established that there is but one cause, viz., molecular motion. 1 The third principle, viz., that, given the cause and no counteracting causes, the effect always happens, is equally necessary to the uniformity of nature. If it were not true physics would be impossible. The fourth principle, that the 1 The application of the molecular theory to heat is one of the most inte- resting examples of the true inductive method. By the liquefaction of two pieces of ice by mutual friction Davy proved that the accepted theory, which identified the objective correlative of the sensation with a subtle fluid (caloric), permeating bodies and forced out of them by friction, could not possibly be true, since a body which had parted with a certain quantity of caloric by fric- tion, as according to the theory the melted ice had done, must require the application of more caloric to raise it to a certain degree of temperature than it required before, and this was not true of the water into which the ice had been converted by the friction, its temperature being higher than. that of the ice. And as the effect of friction was already conceived to be an agitation of the molecules constituting the rubbed body, Davy inferred that such an agitation was the objective correlative of the sensation of heat.