In estimating the strength and the weakness of the system expounded by Lucretius, it is necessary to distinguish between the exposition of the principles of the atomic philosophy, contained in the first two books, and the explanation of natural phenomena contained in the remaining books. The first, notwithstanding some arbitrary and unverifiable assumptions, represents a real and important stage in the progress of enquiry; the second, although containing many striking observations and immediate inferences from the facts and processes of Nature, is, from the point of view of modern science, to be regarded mainly, as a curious page from the records of human error. Whatever may be said of the Epicurean additions to the system, it seems to be admitted that the original hypothesis of Democritus has been more pregnant in results, and has more affinity with the most advanced physical speculations of modern times, than the doctrines of all the other philosophers of antiquity. But even amid the mass of unwarranted assumptions and erroneous explanations contained in the later books, the topics discussed--such as the relation of the mind to the body, the mode by which sensible impressions are conveyed to the mind, the processes by which our globe assumed its present form, the origin of life, the evolution of humanity from its lowest to its higher stages of development, the origin of spiritual beliefs, of the humaner sentiments, of language, etc.--possess the interest of being kindred to those on which speculative activity is most employed in the present day. If the study of Lucretius forces upon our minds the arbitrary assumptions, the inadequate method, and the false conclusions of ancient science, it enables us to appreciate the disinterested greatness of its aims, and the enlightened curiosity which sought to solve the vastest problems. It might be said, generally, that the argument of Lucretius was an attempt to give a philosophical description of Nature before the advent of physical science. But, as a means of throwing light on the inadequacy of such speculations, it may be well to consider in detail some of those points where the argument most obviously fails in premises, method, and results. The ancient as well as the modern enquirer into the truth of things was confronted with the question of the origin of all our knowledge. Is knowledge obtained originally through the exercise of the reason or the senses, or through their combined and inseparable action? To this question Lucretius distinctly answers, that the senses are the foundation of all our knowledge[393]. They are our 'prima fides'; the basis not only of all sound inference, but of all human conduct. The very conception of the meaning of true and false is derived from the senses:-- Invenies primis ab sensibus esse creatam Notitiam veri neque sensus posse refelli.[394]