But the present potential energy of the solar system is so enormous, approaching in fact possibly to what in our helplessness we call infinite, that it may supply for absolutely incalculable future ages what is required for the physical existence of life. Again, the fall together, from the distance of Sirius let us say, of the sun and an equal star would at once supply the sun with at least as much energy for future radiation to possible planets as could possibly have been acquired by his own materials in their original falling together from practically infinite diffusion as a cloud of stones or dust, or a nebula; so that it is certain that, if the present physical laws remain long enough in operation, there will be (at immense intervals of time) mighty catastrophes due to the crashing together of defunct suns—the smashing of the greater part of each into nebulous dust surrounding the remainder, which will form an intensely heated nucleus—then, possibly, the formation of a new and larger set of planets with a proportionately larger and hotter sun, a solar system on a far grander scale than the present. And so on, growing in grandeur but diminishing in number till the exhaustion of energy is complete, and after that eternal rest, so far at least as visible motion is concerned.[43] 116. The study of the necessary future has prepared us for an inquiry into the long remote past. Just as the present discrete stellar systems must finally come together, so the materials which now form them must have originally been widely separate. Our modern knowledge enables us to look back with almost certitude to the time when there was nothing but gravitating matter and its potential energy throughout the expanse of space—ready, as slight local differences of distribution predisposed it, to break up into portions, each converging to one or more nuclei of its own, and thus forming in time separate solar or stellar systems. We have thus reached the beginning as well as the end of the present visible universe, and have come to the conclusion that it began in time and will in time come to an end. Immortality is therefore impossible in such a universe. CHAPTER IV. MATTER AND ETHER. ‘Felix qui potuit rerum cognoscere causas, atque metus omnis et inexorabile fatum subjecit pedibus, strepitumque Acherontis avari.’ VERGIL. ‘Who shall tempt with wandering feet The dark, unbottomed, infinite abyss, And through the palpable obscure find out His uncouth way; or spread his airy flight Over the vast abrupt, ere he arrive The happy isle?’—MILTON, _Paradise Lost_.