The life span of Zeno, the founder of Stoicism, was from B.C. 347 to 275. He did not begin teaching till 315, at the mature age of forty. Aristotle had passed away in 322, and with him closed the great constructive era of Greek thought. The Ionian philosophers had speculated on the physical constitution of the universe, the Pythagoreans on the mystical properties of numbers; Heraclitus had propounded his philosophy of fire, Democritus and Leucippus had struck out a rude form of the atomic theory, Socrates had raised questions relating to man, Plato had discussed them with all the freedom of the dialogue, while Aristotle had systematically worked them out

The Stoics were quite as materialistic in their own way as the Epicureans. With regard indeed to the nature of the highest god we may, with Senaca represent the difference between the two schools as a question of the senses against the intellect, but we shall see presently that the Stoics regarded the intellect itself as being a kind of body.

The Greeks were all agreed that there was an end or aim of life, and that it was to be called 'happiness,' but at that point their agreement ended