_A._ Experience tells us that. I have now done with my problems concerning the great bodies of the world, the stars, and element of air in which they are moved, and am therein satisfied, and the rather, because you have answered me by the supposition of one only motion, and commonly known, and the same with that of Copernicus, whose opinion is received by all the learned; and because you have not used any of these empty terms, sympathy, antipathy, antiperistasis, etc., for a natural cause, as the old philosophers have done to save their credit. For though they were many of them wise men, as Plato, Aristotle, Seneca, and others, and have written excellently of morals and politics, yet there is very little natural philosophy to be gathered out of their writings. _B._ Their ethics and politics are pleasant reading, but I find not any argument in their discourses of justice or virtue drawn from the supreme authority, on whose laws all justice, virtue, and good politics depend. _A._ Concerning this cover, or, as some have called it, the scurf or scab of the terrestrial star, I will begin with you tomorrow. For it is a large subject, containing animals, vegetables, metals, stones, and many other kinds of bodies, the knowledge whereof is desired by most men, and of the greatest and most general profit. _B._ And this is it, in which I shall give you the least satisfaction; so great is the variety of motion, and so concealed from human senses. ========== CHAPTER VII. OF HARD AND SOFT, AND OF THE ATOMS THAT FLY IN THE AIR. _A._ Concerning this cover of the earth, made up of an infinite number of parts of different natures, I had much ado to find any tolerable method of enquiry. But I resolved at last to begin with the questions concerning hard and soft, and what kind of motion it is that makes them so. I know that in any pulsion of air, the parts of it go innumerable and inexplicable ways; but I ask only if every point of it be moved? _B._ No. If you mean a mathematical point, you know it is impossible. For nothing is movable but body. But I suppose it divisible, as all other bodies, into parts divisible. For no substance can be divided into nothings. _A._ Why may not that substance within our bodies, which are called animal spirits, be another kind of body, and more subtile than the common air?