Stoic philosophers initiated a fresh understanding of classical ideals in a world slipping away from its inheritance and feeling its way towards new visions and social structures. Though their efforts at metaphysical subtlety fell short of subsequent neo-Platonic achievements, their deep concern with enacting the philosophical life was unexcelled. Disappointed by the drift of the Academy into scholastic exercises and shunning seductive tendencies to misinterpret the pleasure-centered ethic of Epicurus, the Stoics sought to grasp the full implication of Plato's teachings as exemplified in the Socrates of the Dialogues. The conception of the cosmos as both material and intelligent at every point led naturally to the conclusion that ethics – the principles of conduct for human beings and the laws of motion and interaction for all things – is the architectonic science. Ethics cannot be divorced from either ontology or epistemology, for ethics suffuses cosmic structure and pervades the capacity for conception. All knowledge can be divided into two broad realms, structural (physics) and conceptual (logic). Physics in turn divides into cosmology, the study of universal order, and theology, the discovery of the intelligent maintenance of that order, while logic can be separated into dialectic, the analysis of the rational order of thinking, and rhetoric, the rational principles of persuasion.

Zeno founded the Stoa on original insights into Socratic exemplification, a fundamental rethinking of the Platonic virtues and a bold synthesis of earlier Greek metaphysics, all grounded on the affirmation of the unity of nature, visible and invisible, macrocosmic and microcosmic. Cleanthes drew Zeno's teachings together, elaborated them and gave them a harmonious and lyrical expression. The impact of Stoic philosophy may be measured by the energetic criticism aimed at it, especially by the Cynics from whom the Stoics openly borrowed, the Epicureans who saw the Stoics as rough-hewn and the Academy which found them metaphysically crude. Countered vigorously by Zeno and sympathetically by Cleanthes, the rivalry of schools took its toll, and many initially attracted to the strength of Stoic thought drifted away from the call for endurance. It fell to Chrysippus to establish the school on the bedrock of logic and refined concepts.

Chrysippus was born at Soli in Cilicia about 279 B.C. Little is known of his early life beyond the fact that he grew up in the neighbourhood of Tarsus and perhaps experienced the philosophical disputation of that intellectual centre while still very young. Tradition suggests that he inherited substantial property which was subsequently stolen from him through legal machinations. Finding himself penniless, he embarked for Athens to study philosophy. Though he may have arrived at the Stoa in time to study briefly with Zeno, he certainly heard the discourses of the great teachers of the day – Arcesilaus, Aristo of Chios and Cleanthes. Considering the teachings of Arcesilaus and the Academy to be insipid in their reversion to the preliminary scepticism of Socrates while ignoring the later teachings of Plato, Chrysippus listened intently to Aristo, the most popular teacher in Athens. Aristo had studied under Zeno, but had departed from him and taken up a position closer to the Cynics. Zeno had called him a chatterbox, and many found his personal habits indulgent. He held that logic was barren when not meaningless and that nothing could be known with certitude in physics. Ethics alone was important for Aristo, but he refrained from translating broad ethical principles into practical precepts of any kind. Chrysippus suspected that this stance wedded to rhetoric attracted a large following because it burdened its adherents with few demands for change of attitude or way of life. When asked why he did not become an Aristonian, Chrysippus replied, "Had I followed the many, I would not have become a philosopher."

Cleanthes offered the only positive standpoint in Athenian philosophical circles. His generosity and firmness of conviction, his spiritual and intellectual loyalty to his teacher and his fearless practical conclusions concerning the relevance of principles to daily living attracted and won over Chrysippus. The facile philosophies of the day, reflecting the decadence which results from an emerging yet unassimilated internationalism, enticed many who might have adhered to the Stoa. The gentleness exhibited by Cleanthes in philosophical debate and personal conduct was taken as a sign of weakness, while the deceptive rhetoric of the Aristonians masked their ethical spinelessness, and the sceptical methods of the Academy and the Cynics were mistaken for strength. The followers of the Stoa decreased in numbers in favour of easier ways, and contemporaries of Cleanthes predicted that the painted porch soon would be empty. Chrysippus resolved to end this tragic trend the moment he gave himself to his master. He was completely persuaded as to the truth of the doctrines Cleanthes promulgated, but he was exasperated by the methods employed. Cleanthes the boxer was gracious in speech, whereas Chrysippus, who knew no violent sport, became a verbal pugilist. Though he had the greatest respect for his teacher until the moment of death, he challenged him so harshly that in old age he reproached himself for not having behaved more gently towards Cleanthes. If his request that Cleanthes only provide the doctrines while he himself would furnish the supporting argumentation betrayed an ebullient confidence, his zeal in dialectics earned him the title "the second founder of the Stoa". Diogenes Laertius went so far as to assert: "If there had been no Chrysippus, there would be no Stoa." His chief disappointment seems to have been that, after decisively countering the criticisms of Arcesilaus and Aristo, he outlived the greatest opponents of Stoic thought, only to find himself confronting mediocrity. By the end of his long career as head of the school from 232 B.C. until his death in 206 B.C., the Stoic standpoint was firmly rooted, surviving largely intact while remaining strikingly flexible until the deaths of Epictetus and Marcus Aurelius, its last great spokesman. Aristocreon, the nephew of Chrysippus and a Stoic, erected a statue in his honour as the teacher who could cut through the knots of the philosophers.

Chrysippus plunged energetically into the arena of philosophical disputation at least partly because he thought, unlike some of his opponents, that the outcome mattered. Though of a temperament markedly different from that of Cleanthes, he demonstrated a commensurate willingness to modify his views in the light of the insights of others. His dialectical skill was formidable, but he insisted on stating the case of those who differed with him in the strongest possible terms. He was even criticized by his followers for arguing his opponents' views more forcefully than his own. The Stoic conception of knowledge involved four general elements – sensations, perceptions, conceptions and innate ideas. Elements may be combined into systema, which can be a kind of techne or art, but the systematic synthesis of all knowledge is episteme or science. Philosophy alone constitutes a complete science in this sense. Since innate ideas are difficult to distinguish from amongst the variety of concepts and notions found in human minds, and since most conceptualization involves some link to sensations and perceptions, Zeno preferred a straightforward empirical approach to knowledge. Chrysippus formulated the criterion of truth as kataleptike phantasia, the comprehensive mental image. Reminiscent of the Buddhist idea of vividness of impression, for Chrysippus clarity distinguishes veridical mental images from indistinct and false ones. This distinguishing mark is practical in that it can be applied at the moment of perception, but Chrysippus realized that it cannot stand alone, for in the case of dreams, lack of clarity may be discovered only at some later moment. One might not recognize the clarity of other perceptions due to tendencies and predispositions arising from previous experiences. Chrysippus added that the criteria are sensation and preconception, for perceptions may be judged by the test of reason. He recognized the insights of the correspondence and coherence views of truth and attempted to bring them together.

The Stoic conviction that the universe is a cosmos in the strictest sense and that every aspect of nature and society reflects the activity of the Logos, led Chrysippus to affirm that language is natural, not conventional. This suggests that dialectic should be rooted in logic, for the order of correct reasoning reflects the order of nature. His efforts to refine the principles of logical structure led him to outline the basic undemonstrable forms of propositional logic, including modus ponens, modus tolens and three forms of disjunction. He indicated the parameters of syllogistic reasoning and showed that dialectical demonstration may be simplified through careful definition. Rather than attempt to prove that the universe is an ordered unity, he defined it as "the combination of heaven and earth and all natures that are in them" and "the combination of gods and men and all that is created for their sake", and then argued that it is self-generated, not created by another. This conception of an ordered whole may be identified with Deity, stressing at once the immanent aspect of God and the intelligent aspect of Nature, and insofar as such a conception requires a larger totality from which it is distinguished by its order, that vastitude is the boundless void, unthinkable and unknowable, intimating an utterly transcendent aspect of the Divine.

The universe may be thought of in Heraclitean terms as eternal in its essential nature but perishable in its qualities, the playground of numberless systems of worlds. For Chrysippus as for Cleanthes, the dissolution of any particular system is always fire, the transforming and consuming element which is the most accurate visible symbol of Deity in motion. Sidereal motions and the rhythmic motions of the planets mark the large and small phases of the progress of the cosmos from birth to dissolution. The cohesive force of the universe is centred in the Logos in the cosmos, and so all motion is subject to the principles of universal order. Drawing the conclusion Zeno drew, Chrysippus taught that everything takes place according to fate. Those who sought to mock the austere logic of Chrysippus countered this view with the doctrine of argos logos, laziness. Why do anything if all things are fated? If one is fated to reap the autumnal harvest, why bother sowing in the spring? Chrysippus saw such disputes as purely verbal and relished the task of dissolving them. All things are fated, and some things are fated to occur together or in some recognizable sequence. The teaching of confatalia requires, for instance, that sowing and reaping go together, and one discovers the fated connection through observation. Some connections can be discovered by reason alone, for instance, that sleeping watchers cannot be, or that there is no victory in battle by runaways.

Unlike Cleanthes, Chrysippus thought that every past event is necessary, but that some things are possible even though they are not and never will be true. Fate can rule that something possible, such as the shattering of a large diamond, will never in fact occur. Fate is not a blind, mechanical necessity, but the ordered necessity that is the regulative activity of the pervasive Logos. Fate is determinate only if intelligent action is included in the concept, so that 'fate' is equivalent to 'cosmic reason'. Thus "the essence of fate is a spiritual force, duly ordering the universe". Insofar as the concept of providence has significance distinct from the concept of fate, as Cleanthes taught, Chrysippus insisted, unlike his teacher, that all things fated are also providential. Fate is the Deity. Summing up his position, Chrysippus said:

It is impossible that even the least of particulars can fall out otherwise than in accordance with the will of God, with his word, with law, with justice and with providence.

While the continuing order of the world might be sufficient to suggest the beneficence of Deity, evil seems to challenge the strict view of fate that Chrysippus espoused. He supported his view with a three-pronged analysis of evil. First of all, he reasoned from the interdependence of opposites.

There could be no justice, unless there were also injustice; no courage, unless there were cowardice; no truth, unless there were falsehood.

Secondly, he taught that some seeming evils are necessary consequences of goods, as the vulnerability of the human head to injury is the unavoidable result of its need to be compact and delicately constructed. And thirdly, he acknowledged that one may not have sufficient knowledge to account rationally for some specific evils.

In contrast to the denial that Deity exists, Chrysippus offered three proofs. In an ordered world, reason is the superior power. Since man is not the highest instance of that power, since he did not form the planets, there must be a higher instance, and that is God. Further, human reason must be derived from some source, and that is properly denominated 'Deity'. Finally, if there were no Deity, man would be the supreme being, and the most superficial examination of the universe reveals the arrogance of any such claim. If, however, Deity is the tonos, the magnetic centre of cohesion in the cosmos, the soul is that centre in man, the God in human flesh. The microcosm reflects the macrocosm and so death can only be the separation of soul from body, just as the dissolution of the qualities of the universe is only the separation of manifest existence from the essence of cosmos which is eternal. The separation implies no dualism but only a change of state. Cleanthes taught that souls survive until all distinctions are dissolved in the conflagration which results in the return of the universe to a condition of total non-manifestation or pure potentiality. Chrysippus added that since reason is the soul in man, reason unexercised in life would be indistinguishable from the Logoic dynamics of nature once the individuating qualities of the body vanished, and so only the souls of the wise could properly be said to survive as coherent individualities until the fiery end of time.

Since the universe consists of hosts of intelligent forces, it is a cosmopolis and human society is cosmopolitan only to the extent that human beings collectively and individually discern the order in nature and emulate it. For Zeno this was to live consistently, and Cleanthes taught that it meant living consistently with nature. Chrysippus elaborated the concept of nature to include "universal and human nature", suggesting that the virtues are not simply reflections of the order of nature but rather an integral part of it.

To live virtuously is to live according to scientific knowledge of the phenomena of nature, doing nothing which the universal law forbids, which is the right reason that pervades all things and is the same as Zeus, the lord of the ordering of this world.

The virtuous individual is sensitive to the subtlest movements of nature, is aware of the parameters of his own character and is willing to subordinate personal interests to the requirements of the whole universe. Though virtue is quality of soul, the intimate involvement of soul with body suggests that any deficiency in virtue necessarily affects the tonos of the individual and therefore the health of the body. Chrysippus compared diseases of soul and body, teaching that harmony is as necessary for a healthy mind as it is for a healthy body.

All human beings belong to the Stoic cosmopolis in the most general meaning of that term, but the individual ardently cultivating virtues selects himself for self-conscious participation in that self-chosen cosmopolis of those who emulate the ideal of the wise man and live to serve universal ends. Chrysippus saw the importance of keeping this central ideal absolutely pure and without the slightest demeaning taint, no matter how unconscious, for if any human imperfection or deception casts a shadow on the ideal, the self-conscious cosmopolis is threatened at its very foundations. Thus he was unwilling to acknowledge that more than one or two wise men existed, though many approached the ideal admirably. For the Stoic, the Homeric hero-ideal was far less attractive than the ideal of humanitas, the capacity to reason, sympathize, seek wisdom and sacrifice for the community of man and of nature. With the Academy, the Stoics held that human beings should aim at those ends which are characteristically human and distinct from those of the animal kingdom, and they accepted wisdom, justice, courage and temperance (sophrosyne) as the cardinal virtues. Further, the Stoa taught that the highest human capacities are reason and the political sense, itself an aspect of universal reason. Chrysippus divided virtues into those based on theoretical principles, which are arts (technai) and those evolved from daily practice, which he called attainments (dunameis). According to Seneca, these included, in addition to humanitas, fortitudo, fides, temperantia, simplicitas, modestia and moderatio, frugalitas and parsimonia and dementia. Justice derives from political understanding and is critical to the flourishing cosmopolis. Chrysippus allegorized this virtue as a virgin.

She is of virgin form, to show that she is incorruptible and does not give way to bad men; of firm and fierce aspect, inspiring fear in the wicked and confidence in the good. Her eyes are keen-sighted, her bearing is at once sad and awe-inspiring.

Nobility cannot be a matter of birth, for since all individuals derive by degrees from the same divine origin, though few may easily manifest their spiritual ancestry, true nobility is the exemplification of virtue, a station accessible to all who will strive to assume it. The good man will not lift a finger for his reputation unless some larger purpose is involved. But, Chrysippus added, one is bound to avoid whatever undermines virtue, such as drunkenness.

Through his detailed study of cosmic harmony, fate and virtue, Chrysippus elaborated a teaching which partly echoes the Hindu conceptions of Rta, karma and dharma. While some of his followers were inclined to draw up lists of permissible activities and social taboos, Chrysippus jokingly remarked that "the wise man will turn three somersaults for a sufficient fee", thereby indicating that no rigid rules should be laid down except that one should seek a living which does not injure one's neighbour. He refused the offer of political office, saying, "if a man is a bad politician, he is hateful to the gods; if a good politician, to his fellow-citizens". He turned his attention to systematizing and clarifying the teachings he had learnt from his master Cleanthes. He wrote seven hundred and five treatises to explain and defend these precious doctrines, and though none survives, the school he headed from the death of his teacher until his own death in 206 B.C. was given the strength and influence to flourish for almost four centuries. He earned the title Column of the Portico and was honoured as the second founder of the Stoa. Perhaps the most generous tribute was offered by Diogenes Laertius, who said:

If the gods use dialectic, they can use none other than that of Chrysippus.

Embued with pure discrimination, restraining himself with resolution, having rejected the charms of sound and other objects of the senses, and casting off attachment and dislike; dwelling in secluded places, eating little, with speech, body, and mind controlled, engaging in constant meditation and unwaveringly fixed in dispassion; abandoning egotism, arrogance, violence, vanity, desire, anger, pride, and possession, with calmness ever present, a man is fitted to be the Supreme Being.