If you strip Stoicism of its paradoxes and its wilful misuse of language, what is left is simply the moral philosophy of Socrates, Plato and Aristotle, dashed with the physics of Heraclitus. Stoicism was not so much a new doctrine as the form under which the old Greek philosophy finally presented itself to the world at large. It owed its popularity in some measure to its extravagance. A great deal might be said about Stoicism as a religion and about the part it played in the formation of Christianity but these subjects were excluded by the plan of this volume which was to present a sketch of the Stoic doctrine based on the original authorities.
Philosophy was defined by the Stoics as 'the knowledge of things divine and human'. It was divided into three departments; logic, ethic, and physic. This division indeed was in existence before their time, but they have got the credit of it as of some other things which they did not originate. Neither was it confined to them, but was part of the common stock of thought. Even the Epicureans, who are said to have rejected logic can hardly be counted as dissentients from this threefold division. For what they did was to substitute for the Stoic logic a logic of their own, dealing with the notions derived from sense, much in the same way as Bacon substituted his Novum Organum for the Organon of Aristotle. Cleanthes we are told recognised six parts of philosophy, namely, dialectic, rhetoric, ethic, politic, physic, and theology, but these are obviously the result of subdivision of the primary ones. Of the three departments we may say that logic deals with the form and expression of knowledge, physic with the matter of knowledge, and ethic with the use of knowledge. The division may also be justified in this way. Philosophy must study either nature (including the divine nature) or man; and, if it studies man, it must regard him either from the side of the intellect or of the feelings, that is either as a thinking (logic) or as an acting (ethic) being.