This double experience of mutation and recurrence, an experience at once sentimental and scientific, soon brought with it a very great thought, perhaps the greatest thought that mankind has ever hit upon, and which was the chief inspiration of Lucretius. It is that all we observe about us, and ourselves also, may be so many passing forms of a permanent substance. This substance, while remaining the same in quantity and in inward quality, is constantly redistributed; in its redistribution it forms those aggregates which we call things, and which we find constantly disappearing and reappearing. All things are dust, and to dust they return; a dust, however, eternally fertile, and destined to fall perpetually into new, and doubtless beautiful, forms. This notion of substance lends a much greater unity to the outspread world; it persuades us that all things pass into one another, and have a common ground from which they spring successively, and to which they return. The spectacle of inexorable change, the triumph of time, or whatever we may call it, has always been a favourite theme for lyric and tragic poetry, and for religious meditation. To perceive universal mutation, to feel the vanity of life, has always been the beginning of seriousness. It is the condition for any beautiful, measured, or tender philosophy. Prior to that, everything is barbarous, both in morals and in poetry; for until then mankind has not learned to renounce anything, has not outgrown the instinctive egotism and optimism of the young animal, and has not removed the centre of its being, or of its faith, from the will to the imagination. To discover substance, then, is a great step in the life of reason, even if substance be conceived quite negatively as a term that serves merely to mark, by contrast, the unsubstantiality, the vanity, of all particular moments and things. That is the way in which Indian poetry and philosophy conceived substance. But the step taken by Greek physics, and by the poetry of Lucretius, passes beyond. Lucretius and the Greeks, in observing universal mutation and the vanity of life, conceived behind appearance a great intelligible process, an evolution in nature. The reality became interesting, as well as the illusion. Physics became scientific, which had previously been merely spectacular.