Here was a much richer theme for the poet and philosopher, who was launched upon the discovery of the ground and secret causes of this gay or melancholy flux. The understanding that enabled him to discover these causes did for the European what no Indian mystic, what no despiser of understanding anywhere, suffers himself to do; namely, to dominate, foretell, and transform this changing show with a virile, practical intelligence. The man who discovers the secret springs of appearances opens to contemplation a second positive world, the workshop and busy depths of nature, where a prodigious mechanism is continually supporting our life, and making ready for it from afar by the most exquisite adjustments. The march of this mechanism, while it produces life and often fosters it, yet as often makes it difficult and condemns it to extinction. This truth, which the conception of natural substance first makes intelligible, justifies the elegies which the poets of illusion and disillusion have always written upon human things. It is a truth with a melancholy side; but being a truth, it satisfies and exalts the rational mind, that craves truth as truth, whether it be sad or comforting, and wishes to pursue a possible, not an impossible, happiness. So far, Greek science had made out that the world was one, that there was a substance, that this was a physical substance, distributed and moving in space. It was matter. The question remained, What is the precise nature of matter, and how does it produce the appearances we observe? The only answer that concerns us here is that given by Lucretius; an answer he accepted from Epicurus, his master in everything, who in turn had accepted it from Democritus. Now Democritus had made a notable advance over the systems that selected one obvious substance, like water, or collected all the obvious substances, as Anaxagoras had done, and tried to make the world out of them. Democritus thought that the substance of everything ought not to have any of the qualities present in some things and absent in others; it ought to have only the qualities present in all things. It should be _merely_ matter. Materiality, according to him, consisted of extension, figure, and solidity; in the thinnest ether, if we looked sharp enough, we should find nothing but particles possessing these properties. All other qualities of things were apparent only, and imputed to them by a convention of the mind. The mind was a born mythologist, and projected its feelings into their causes. Light, colour, taste, warmth, beauty, excellence, were such imputed and conventional qualities; only space and matter were real. But empty space was no less real than matter. Consequently, although the atoms of matter never changed their form, real changes could take place in nature, because their position might change in a real space.