It has been the fashion in certain quarters to look down on these early thinkers—to depreciate the value of their speculations because they were thinkers, because, as we have already noticed, they reached their most important conclusions by thinking, the means of truly scientific observation not being within their reach. Nevertheless, they performed services to humanity comparable for value with the legislation of Solon and Cleisthenes, or the victories of Marathon and Salamis; while their creative imagination was not inferior to that of the great lyric and dramatic poets, the great architects and sculptors, whose contemporaries they were. They first taught men to distinguish between the realities of nature and the illusions of sense; they discovered or divined the indestructibility of matter and its atomic constitution; they taught that space is infinite, a conception so far from being self-evident that it transcended the capacity of Aristotle to grasp; they held that the seemingly eternal universe was brought into its present form by the operation of mechanical forces which will also effect its dissolution; confronted by the seeming permanence and solidity of our planet, with the innumerable varieties of life to be found on its surface, they declared that all things had arisen by differentiation[8] from a homogeneous attenuated vapour; while one of them went so far as to surmise that man is descended from an aquatic animal. But higher still than these fragmentary glimpses and anticipations of a theory which still awaits confirmation from experience, we must place their central doctrine, that the universe is a cosmos, an ordered whole governed by number and law, not a blind conflict of semi-conscious agents, or a theatre for the arbitrary interference of partial, jealous, and vindictive gods; that its changes are determined, if at all, by an immanent unchanging reason; and that those celestial luminaries which had drawn to themselves in every age the unquestioning worship of all mankind were, in truth, nothing more than fiery masses of inanimate matter. Thus, even if the early Greek thinkers were not scientific, they first made science possible by substituting for a theory of the universe which is its direct negation, one that methodised observation has increasingly tended to confirm. The garland of poetic praise woven by Lucretius for his adored master should have been dedicated to them, and to them alone. His noble enthusiasm was really inspired by their lessons, not by the wearisome trifling of a moralist who knew little and cared less about those studies in which the whole soul of his Roman disciple was absorbed.