Varro and Pliny were encyclopaedists. Yet before, as well as after them, the men possessed by the passion for knowledge of the natural world, were frequently devoted to some branch of inquiry, rather than encyclopaedic gleaners, or universal philosophers. Hippocrates, Socrates’s contemporary, had left a name rightly enduring as the greatest of physicians. In the third century before Christ Euclid is a great mathematician, and Hipparchus and Archimedes have place for ever, the one among the great astronomers, the other among the great terrestrial physicists. All these men represent reflection and theory, as well as investigation and experiment. Leaping forward to the second century A.D., we find among others two great lovers of science. Galen of Pergamos was a worthy follower, if not a peer, of the great physician of classic Greece; and Ptolemy of Alexandria emulated the Alexandrian Hipparchus, whose fame he revered, and whose labours (with his own) he transmitted to posterity. Each of these men may be regarded as advancing some portion of the universal plan of Aristotle. Another philosophy, Stoicism, had already reached a wide acceptance. As for the causes of this, doubtless the decline of Greek civic freedom before the third century B.C., had tended to throw thoughtful men back upon their inner life; and those who had lost their taste for the popular religion, needed a philosophy to live by. Stoicism became especially popular among the Romans. It was ethics, a philosophy of practice rather than of knowledge. The Stoic looked out upon the world from the inner fortress of the human will. That guarded or rather constituted his well-being. He cared for such knowledge, call it instruction rather, as would make good the principle that human well-being lay in the rightly self-directing will. He did not seriously care for metaphysics, or for knowledge of the natural world, save as one or the other subserved the ends of his philosophy as a guide of life. Thus the Stoic physics, so important a part in the Stoic system, was inspired by utilitarian motives and deflected from unprejudiced observation by teleological considerations and reflections on the dispensations of Providence. Of course, some of the Stoics show a further range of intellectual interest; Seneca, for example, who was a fine moralist and wrote beautiful essays upon the conduct of life. He, like a number of other people, composed a book of _Quaestiones naturales_, which was chiefly devoted to the weather, a subject always very close to man. But he was not a serious meteorologist. For him the interest of the fact lay rather in its use or in its moral bearing. After Seneca the Stoic interest in fact narrows still further, as with Epictetus and Marcus Aurelius.