The history of the physical science of the ancients cannot, indeed, be regarded as so interesting or important as that of their metaphysical philosophy. And this is so, not only on account of the comparative scantiness of their real acquisitions in the one as compared with the ideas and method which they have contributed to the other, and with the masterpieces which they have added to its literature; but still more on this account, that in physical knowledge new discovery supplants the place of previous error or ignorance, and can be understood without reference to what has been supplanted; whereas the power and meaning of philosophical ideas is unintelligible, apart from the knowledge of their origin and development. The history of physical science in ancient times affords satisfaction to a natural curiosity, but is not an indispensable branch of scientific study. The history of ancient mental philosophy, on the other hand,--the source not only of most of our metaphysical ideas and terms, but of many of the most familiar thoughts and words in daily use,--is the basis of all speculative study. Yet among the various kinds of interest which this poem has for different classes of modern readers this is not to be forgotten, that it enables a student of science to estimate the actual discoveries, and, still more, the prognostications of discovery attained by the irregular methods of early enquiry. The school of philosophy to which Lucretius belonged was distinguished above other schools for the attention which it gave to the facts of Nature. Though he himself makes no claim to original discovery, he yet shows a philosophical grasp of the whole system which he adopted, and a rigorous study of its details. He does not, like Virgil, merely reproduce some general results of ancient physics, to enhance the poetical conception of Nature: as he is not satisfied with those general results about human life and the origin of man, which amused a meditative poet and practical epicurean like Horace. He was a real student both of the plan of Nature and of man's relation to it. Out of the stores of his abundant information the modern reader may best learn not only the errors but also the happy guesses and pregnant suggestions of ancient science. To the general reader there is another aspect, in which it is interesting to compare these germs of physical knowledge with some tendencies of scientific enquiry in modern times. The questions, vitally affecting the position of man in the world, which are discussed or raised by Lucretius in the course of his argument, are parallel to certain questions which have risen into prominence in connexion with the increasing study of Nature. Most conspicuous among these is the relation of physical enquiry to religious belief. Expressions such as this, Impia te rationis inire elementa viamque Indugredi sceleris,