PLACE OF HYPOTHESIS IN EXPEEIMENTAL SCIENCE. 561

sum total of ultimate causes is a fixed quantity, follows from the first ; for the coming into being of a new ultimate cause, or the passing out of existence of an old one, would be an uncaused event. The fifth principle, that of the reciprocal action of causes, is essential to the idea of an universe. If any cause or set of causes was unrelated to the rest, there would be, strictly speaking, no universe. If, e.g., the forces which make for rest and those which make for motion were unrelated, there would be two worlds but no universe a world in which everything was at rest and a world in which change was perpetual and universal, but no transition from motion to rest and rest to motion. There would thus be no universe, no organic unity of things. It is clear therefore that the existence of the universe implies that the statical and dynamical forces do interact. It is further implied that such forces are equal in amount. If, e.g., there were a pre- ponderance of the statical over the dynamical forces, the universe must eventually pass into a state of complete quies- cence. This would, however, involve the impossible supposi- tion of a last event. It follows that the statical and dynami- cal forces must be regarded as equivalent. If, however, they were also equally distributed, it is plain that no change could ever occur. Hence we must regard them as equivalent indeed but unequally distributed. It is further clear that, if this inequality of distribution were very great, the universe would present the aspect of a series of cataclysmic changes, instead of the incessant but gradual change which we know and which is implied in the uniformity of nature. Nor is it possible to conceive that the universe will ever enter upon a period of general and continuous cataclysmic change. The violence of change is proportionate to the resistance offered ; hence every cataclysm presupposes a period during which the statical forces have been largely predominant, during which the dynamical forces have slowly accumulated. No rapid and general succession of cataclysmic changes is there- fore possible. These principles constitute, in my opinion, what we mean by the uniformity of nature and the immutability of law. They are indispensable to physics, but they are not empiri- cally verifiable ; the only verification of which they are sus- ceptible is just their indispensability. They are principles of the possibility of physical science. Laws of causation, on the other hand, are figurative schemata whereby the essen- tial unity of the universe is bodied forth to the eye of imagi- nation. As such they are not devoid of truth, and indeed of positive truth. An idealist who holds that matter and force