(5) The introduction of letters was closely followed by the rise of Philosophy, or the reflective spirit. Up to about the year 600 B.C., the Greeks, like the rest of the world, lived by habit, tradition, and prescription, handed on, with little or no criticism, from generation to generation. Their ideal world was shaped by the works of Homer and Hesiod. "Hesiod is the teacher of most," says Heraclitus. About the date named, however, society having advanced to a condition of organization which made possible a leisure class, there begins to appear a new spirit, destined to revolutionize, not only Greece, but the whole world. Armed with a _what?_ a _which?_ a _why?_ and a _wherefore_? it no longer blindly accepts the world of nature and man, but calls upon it to give an account of itself. Science, philosophy, and art are the result. At first the new spirit turns to nature with a _what?_; but, gradually discovering that the answer to this brings no complete explanation of the world, it propounds its other questions. It thus arrives at a consciousness of four distinguishable elements in the constitution of things,--four causes (αἴτια, αἰτίαι), as they were termed,--(1) matter, (2) form, (3) efficiency, (4) end or aim. At the same time, and by the same process, it is forced to a recognition of the presence of reason (λόγος) and intelligence (νοῦς) in the world, since form, efficiency, and aim all presuppose both. It is thus compelled to turn from nature to man, and man's mind, as the highest known expression of reason and intelligence, and to devote itself to the consideration of spirit, as alone promising any true explanation of the world. The process is a slow and difficult one, and the history of it is the history of Greek science, philosophy, and art. Before the rise of philosophy, the teacher of the people had been the rhapsode, or public reciter; after that event he gradually gives place to the sophist (σοφιστής, one who makes wise), or, as he later with more modesty calls himself, the philosopher (φιλόσοφος, lover of wisdom). The history of Greece for centuries is, on its inner side, a history of the struggle between what the rhapsode represents and what the philosopher represents, between popular tradition and common sense on the one hand, and individual opinion and philosophy on the other. The transition from the first to the second of these mental conditions was accomplished for the world, once for all, by the Greeks, and the turning-point in the process is marked by