=105.= Whilst the works of Chrysippus cover the whole range of the Stoic philosophy, their special colour is largely due to the interests of his own time. The stress laid by Zeno on the certainty of knowledge had produced a reaction in the Academic school. Arcesilaus, who had succeeded Polemo as its leader, leaving on one side the positive teaching of Plato’s later years, reverted to the sceptical attitude which had been one characteristic of Socrates, and which is so prominent in most of the Platonic dialogues[98]. He attacked with the utmost vigour Zeno’s doctrine of ‘comprehension’; and further argued that certain knowledge is unnecessary for practical life, of which probability, that is, such action as can find reasonable justification, is the sufficient guide[99]. Chrysippus defended with the utmost energy the dogma of the certainty of knowledge, based upon the perspicuity of true mind pictures[100]; but the teaching of Arcesilaus obtained a hold upon him, and (as we shall see) was ultimately allowed by him a place within the Stoic system. [Sidenote: Spread of Epicureanism.] =106.= Chrysippus meanwhile had a more dangerous enemy to meet than the Academy. During the weakness which befel the Stoic school in the middle of the third century B.C., the rival school of Epicurus had won an enormous popularity. Yet its ethical standard, which it had inherited from the Cyrenaics, offended not only the followers of Zeno but all sober-minded philosophers. For Epicurus had set up Pleasure as the queen of life, and had converted the virtues into her handmaidens[101]; and so far was he from taking interest in model states, that he advised his hearers to hold aloof altogether from public life. Worst of all, his followers only smiled at the reproofs that were showered upon them. They formed among themselves a cheerful, affectionate, and united society; their simple pleasures created no public scandal, though their entertainments were often enlivened by tales of the moral lapses of their self-righteous rivals. The bracing morality of Cynism seemed to be quite gone out of fashion, and even the Aristonians had ceased to exist. [Sidenote: Alliance of the three schools.]