A few out of many erroneous assertions about physical facts, in regard to some of which the opinions of Lucretius are behind the science even of his own time, may be noticed. Thus, at i. 1052, the existence of the Antipodes is denied. Again, in Book iii. the mind is stated to be a material substance, seated in the centre of the breast, composed of very minute particles, the relative proportions of which determine the characters both of men and animals. Lucretius shows a close and subtle observation of facts that establish the interdependence of mind and body, but no suspicion of that interdependence being connected with the functions of the brain and nervous system. His whole account of the _mundus_, of the earth at rest in the centre, and of the rolling vault of heaven, with its sun and moon and stars--'trembling fires in the vault'--all no larger than they appear to our eyes, is given without any notion of the inadequacy of his data to bear out his conclusions. The science which satisfied Epicurus was on astronomical and meteorological questions behind that attained by the mathematicians of Alexandria: and thus some of the conclusions enunciated by Virgil in the Georgics are nearer the truth than those accepted by Lucretius. While enlarging on the variety and subtlety in the combinations of his imaginary atoms, he has no adequate idea of the variety and subtlety in the real forces of Nature. His observation of the outward and visible appearances of things is accurate and vivid: there is often great ingenuity as well as a true apprehension of logical conditions in his processes of reasoning both from ideas and from phenomena: yet most of his conclusions as to the facts of Nature, which are not immediately perceptible to the senses, are mere fanciful explanations, indicating, indeed, a lively curiosity, but no real understanding of the true conditions of the enquiry. The root of his error lies in his not feeling how little can be known of the processes and facts of Nature by ordinary observation, without the resources of experiment and of scientific method built upon experiment.