26 THOMAS WHITTAKEB : most important ancient phase was Atomism. The physics of Democritus and Epicurus, ready to the hand of scientific philosophers at the opening of the modern era, grew into the corpuscular Mechanics of the seventeenth century. Taken up again by Dalton from Newton, it received its most ac- curate and verifiable expression in the atomic theory of modern Chemistry. Meanwhile, with Descartes and the Cartesian school, there had come into clear view for the first time the idea of formulating a law of indestructibility of motion, as it was then put. For motion" or momentum, Leibniz substituted vis viva or "force". At length, in the nineteenth century, the anticipated law was accurately for- mulated as the law of the Conservation of Energy. That Matter and Energy are alike perdurable through all change is not, however, sufficient for scientific uniformity. A law of sequence among the changes themselves is also needed. This has been expressed as the Law of Causation, and, in this expression, has been made a fundamental principle of Inductive Logic. In the modern development of the logic of Induction, the great names are those of Bacon, Hume, Comte and Mill. Since Mill, we have a logic of the investi- gation of nature comparable, in its systematic character, with the formal logic of Aristotle. In their investigation of the subjective grounds of the principle of Uniformity, Hume and Mill applied themselves more specially to the philosophical or metaphysical problem. To Bacon must be ascribed distinctively the idea of methodical induction, in contrast with " induction by simple enumera- tion," and to Comte the idea of a scientifically certain or positive " law " of phenomena. On the metaphysical question there is now perhaps more agreement among philosophers than appears. Experientialists do not uphold Mill's view that the Uniformity of Nature is itself established by an induction from particulars ; and the successors of Kant on their side do not think that experience can be constituted by mental forms or "categories" applied to a chaos of given sensations. Kant's position as against Hume being con- ceded to this extent, that experience has its formal elements which are as real as the matter of perception, Kantians or Hegelians hardly contend for more. The categories, they themselves allow, are immanent in experience, and do not need to be imposed on it from without. Indeed the notion that Hume was a pure sceptic without serious belief in scientific truth, or that Kant held nature to be a chaos put in order by the individual human mind, would be allowed to be too " schematic," and not agreeable to the deeper drift of
