554 J. M. RIGG- :

given hypothesis. Physics assumes what empirical philo- sophy, if it does not deny, at least will not assert, viz., the existence of a world of noumenal causes and " real essences"; and it is with the attempt to define the nature of these noumenal causes and real essences that induction in the strict sense begins. It is probably because they come to the subject with minds clouded by sensational metaphysics that logicians of the empirical school have overlooked this fact. They have described rather the method of their own philo- sophy than that of physical science, which is not so much empirical as theoretic. Had the aim of the early physicists been merely to observe and tabulate uniformities of relation, it is not too much to say that neither astronomy nor physics would have come into being. By the method of observing and registering relations of coexistence, antecedence and sequence alone, we could never have come by the knowledge even of the sun, moon and stars. Had causation meant for the early thinkers invariability of antecedence and sequence (and how could they have distinguished between condi- tional and unconditional invariability?), they would have noted the fact that the emergence of a luminous disc from below the horizon was the uniform antecedent of day and its disappearance the uniform antecedent of night, and there the matter must have ended. The hypothesis that both phenomena were occasioned by the motion of a vast body so distant from the earth as to present to them the appearance of a mere disc of light could only have been framed by men who believed in a world transcending experience and sought to define it. Pythagoras in substituting the idea of the sun as the centre of the planetary system for that of the earth, the Alexandrian astronomers in reconciling the appa- rent motions of the sun and planets with the immobility of the earth by their elaborate theory of epicycles and ec- centrics, Copernicus in showing that the Pythagorean theory, while simpler than the Ptolemaic, explained at once the appa- rent motions of the sun and planets and the apparent immo- bility of the earth, all employed a method which was cer- tainly inductive, but at the same time boldly set at nought the testimony of the senses and transcended experience. The motions of the planets are no more phenomena than are the ultimate atoms of which their substance is supposed to be constituted ; they are not empirically verifiable, but we know that if they take place they must present to us the appearances which we see, and we assume their existence because we are unable to deduce the phenomena from any