energy of each chosen centre, as well as its career and encounters, hang on the collateral existence of other centres of force, among which it must wend its way: yet the only witness to their presence, and the only known property of their substance, is their "radio-activity", or the physical light which they shed. Light, in its physical being, is accordingly the measure of all things in this new philosophy: and if we ask ourselves why this element should have been preferred, the answer is not far to seek. Light is the only medium through which very remote or very minute particles of matter can be revealed to science. Whatever the nature of things may be intrinsically, science must accordingly express the universe in terms of light. These reforms have come from within: they are triumphs of method. We make an evident advance in logic, and in that parsimony which is dear to philosophers (though not to nature), if we refuse to assign given terms and relations to any prior medium, such as absolute time or space, which cannot be given with them. Observable spaces and times, like the facts observed in them, are given separately and in a desultory fashion. Initially, then, there are as many spaces and times as there are observers, or rather observations; these are the specious times and spaces of dreams, of sensuous life, and of romantic biography. Each is centred here and now, and stretched outwards, forward, and back, as far as imagination has the strength to project it. Then, when objects and events have been posited as self-existent, and when a "clock" and a system of co-ordinates have been established for measuring them, a single mathematical space and time may be deployed about them, conceived to contain all things, and to supply them with their respective places and dates. This gives us the cosmos of classical physics. But this system involves the uncritical notion of light and matter travelling through media previously existing, and being carried down, like a boat drifting down stream, by a flowing time which has a pace of its own, and imposes it on all existence. In reality, each "clock" and each landscape is self-centred and initially absolute: its time and space are irrelevant to those of any other landscape or "clock", unless the objects or events revealed there, being posited as self-existent, actually coincide with those revealed also in another landscape, or dated by another "clock". It is only by travelling along its own path at its own rate that experience or light can ever reach a point lying on another path also, so that two observations, and two measures, may coincide at their ultimate terms, their starting-points or their ends. Positions are therefore not independent of the journey which terminates in them, and thereby individuates them; and dates are not independent of the events which distinguish them. The flux of existence comes first: matter and light distend time by their pulses, they distend space by their deployments.