An intense practicality was a mark of the later Greek philosophy. This was common to Stoicism with its rival Epicureanism. Both regarded philosophy as 'the art of life,' though they differed in their conception of what that art should be. Widely as the two schools were opposed to one another, they had also other features in common. Both were children of an age in which the free city had given way to monarchies, and personal had taken the place of corporate life. The question of happiness is no longer, as with Aristotle, and still more with Plato, one for the state, but for the individual. In both schools the speculative interest was feeble from the first, and tended to become feebler as time went on. Both were new departures from pre-existent schools. Stoicism was bred out of Cynicism, as Epicureanism out of Cyrenaicism. Both were content to fall back for their physics upon the pre-Socratic schools, the one adopting the firm philosophy of Heraclitus, the other the atomic theory of Democritus. Both were in strong reaction against the abstractions of Plato and Aristotle, and would tolerate nothing but concrete reality. 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. As to the nature of happiness there was the utmost variety of opinion. Democritus had made it consist in mental serenity, Anaxagoras in speculation, Socrates in wisdom, Aristotle in the practise of virtue with some amount of favour from fortune, Aristippus simply in pleasure. These were opinions of the philosophers. But, besides these, there were the opinions of ordinary men, as shown by their lives rather than by their language. Zeno's contribution to thought on the subject does not at first sight appear illuminating. He said that the end was 'to live consistently,' the implication doubtless being that no life but the passionless life of reason could ultimately be consistent with itself. Cleanthes, his immediate successor in the school, is credited with having added the words 'with nature,' thus completing the well-known Stoic formula that the end is 'to live consistently with nature.'