There are never any occasions when you need think yourself safe because you wield the weapons of Fortune: fight with your own. Fortune does not furnish arms against herself and so men equipped against their foes are unarmed against Fortune herself.

Seneca, Epistle 113 POSIDONIUS

After Socrates, Plato and Aristotle, Athens gradually ceased to be the philosophical centre of the Mediterranean world. Her importance as a city of learning remained for centuries, but the diffusion of culture, the rise of Alexandria and the growth of Roman power overshadowed the political and economic importance of the polis and eclipsed the social isolation needed to make one place the main focus of intellectual activity. The Stoics were attracted to Athens, but their great teachers came from outside the narrow Hellenic sphere. Zeno arrived from Citium on Cyprus, and was succeeded in turn by Cleanthes of Assos and Chrysippus of Soli, sons of Asia Minor. The Stoic school – which was more of a philosophical tradition than an institution like the Academy or the Lyceum – quietly continued after the passing of this brilliant triad, and when it flourished again as the Middle Stoa, its luminaries did not think of Athens as the sole platform for philosophical discourse. The Late Stoa, led by Seneca, Epictetus and Marcus Aurelius, was entirely Roman.

When the Athenians sent a deputation to Rome to seek relief from a fine imposed upon them, the Stoic Diogenes of Seleucia was a member, along with a Peripatetic and a Sceptic. Cato disliked all of them because their philosophical attitudes tended to diminish the importance of military virtue. Nonetheless, the seeds of Stoic thought were sown in Rome and found fertile ground there. Panaetius of Rhodes, a wealthy man who had studied at the grand library at Pergamum under its head, Crates the Stoic, became a student of Diogenes in Athens. Within a decade of the deputation, Panaetius journeyed to the political centre of the empire. He met and became fast friends with Scipio the Younger, and soon scholars and intellectuals formed a circle around them. Panaetius remained in Rome until the death of Scipio, then he returned to Athens and assumed leadership of the Stoic school.

The popularity of Panaetius was due in part to his urbanity and convivial nature. He was influenced by the philosophical Scepticism of the time and rejected a number of Old Stoic views on the nature of the universe and the importance of logic in discursive thought. He expounded the Stoic conviction that virtue is knowledge and the only good a human being requires, but he softened the asceticism of his philosophical forebears. Whilst honouring the ideal of the wise man, he emphasized the concept of ho prokopton, the probationer engaged in the performance of duty. Thus he added to the Stoic conception of wisdom the possibility of moral growth. As a corollary, he enriched the Stoic ideas of self-knowledge and self-restraint with the broad notion of prepon, often translated 'decorum' but implying mutual harmony in all aspects of one's life. The rather unorthodox Stoicism of Panaetius deeply impressed the Romans, and his views profoundly influenced the Late Stoa. After his return to Athens, Panaetius led the Stoa for almost twenty years. His most illustrious student was Posidonius, who would bring a fresh depth and rigour to the Stoic tradition and who was the crown of non-Roman Stoic thought.

Almost nothing is known of the life of Posidonius despite the enormous fame he achieved in his lifetime and the lasting influence he had on subsequent philosophy. Born around 135 B.C., Posidonius reached manhood in Apamea, a city in northern Syria, and his wealth and intelligence furnished him with an excellent education. His writing was gifted, and though the style he effectively mastered passed out of fashion shortly after his death, his ability to wed word and thought was highly respected for a long time. He travelled to Athens, where he became the pupil of Panaetius, and although the length of his studies is not known, he was universally recognized as the philosophical heir of his teacher. He chose, however, not to assume the Stoic chair and settled in Rhodes. Alexandria had experienced a turbulent period, the results of which included the diffusion of Alexandrian learning throughout the Mediterranean. Many scholars and scientists went into exile at Rhodes, where a famous school of rhetoric flourished. The cosmopolitan atmosphere of the area appealed to Posidonius, who found himself at home amongst the various philosophical and scientific perspectives which could be found there.

Though unusual in his day, he was made a citizen of his adopted home. In contrast to the early Stoic tendency to withdraw from the vagaries of public life, Posidonius became a prominent political figure. He once held the high office of prytanis, and was part of an embassy sent to Rome in 86 B.C. Most of his extensive travels, however, were not for reasons of state. His typically Stoic devotion to truth brought him to question Stoic orthodoxies along with the accepted truths of other traditions. On the philosophical level, he accepted a number of critical arguments elaborated by the Sceptics and remained open to a wide range of Platonic perspectives. Within the context of the Hellenistic and Alexandrian traditions of science, he sought to verify assertions about natural phenomena and, more than other thinkers of the time, to determine their causes. Besides his visit to Rome as a delegate in the Rhodian mission, he made at least one extensive tour of the Western Mediterranean, visiting Sicily and Liguria, a rustic and rough land. He penetrated Gaul and sojourned beyond the Roman boundaries. There he saw tribal practices which would have stunned a less adventurous traveller, and his powers of observation permitted him to recognize the reverence with which Druids were treated. Even there, he noted, "pride and passion give way to wisdom, and Ares stands in awe of the Muses". He tarried in Gades (now Cadiz) to watch the Atlantic tides wash the Spanish shore, and there he discerned the connection of the moon with the size of the tides. He estimated the circumference of the earth with reasonable accuracy and held that the earth's equatorial regions were inhabitable.

Despite their immediate popularity and praiseworthy style, none of his writings survive. Later writers quote him, summarize his books and admire his clarity and originality, but Roman Stoicism followed a different path, the Hellenistic world sank into decline and the growing Church ignored him. He composed a History of the world in fifty-two volumes, beginning where Polybius had ended in 145 B.C. His approach did not aim at mere factuality; he saw in the sweep of history the unfolding of human psychology, individual and collective. His conviction that aristocratic rule constituted the best form of government assured him that Roman dominance was justifiable, but he believed that moral decay was destroying the social order. He also wrote on anthropology. Recognizing that the sun is a pure fire of dimensions far greater than those of the earth, he held that different races arose from its life-giving vital force because the incidence of its angle of radiation varied with the different latitudes humans occupy. Eclectic in his interests, Posidonius nonetheless arranged all his thoughts with a fundamentally Stoic view of the world.

Following in the footsteps of his Stoic predecessors, Posidonius divided philosophy into three general fields: physics, ethics and logic. Most of his physics and logic are lost, but his ethics survive in part because of its continuity with the whole Stoic tradition. Enough remains, however, to reveal a philosophy which altered physics and revolutionized psychology in a way that linked them and changed the Stoic understanding of the world. Philosophy as a discipline, Posidonius taught, has as its aim the formulation of general and universal principles upon which all knowledge is based. No more than blood, organs and skeleton can be separated in a living being without irreparable harm, so the branches of philosophy are mutually interdependent. Like earlier Stoics, Posidonius held that two principles, active and passive, pervade the kosmos. Since the universe is material, these principles represent two features of that materiality – which is not matter in any ordinary experiential sense. The active principle is pneuma (which originally meant 'breath' and implied intelligent activity with the Stoics) and it is conjoined with passive matter at every point. The balance between the two principles gives matter its varying tonos or tensility. Matter without qualities – pure passivity – is never experienced and is therefore a mental construct. Matter always has qualities, thus demonstrating in its very presence the confluence of the active and passive principles.

That which gives quality to matter is pneuma, which can be called God. "God is intellectual pneuma extending throughout the whole substance", Posidonius taught. Similarly, pure activity has no form. Pneuma assumes form through being linked with the passive principle. Thus, "God is intellectual pneuma like fire. He has no form, but he can be altered into what he wants and can become equal to what he wants." Amongst the earlier Stoics, such a view might result in a kind of pantheism from which an ethic of living in harmony with Nature could be derived. Posidonius, however, was satisfied with nothing less than a view that could explain both the variations of tonos and the psychological roots of inharmonious and irrational behaviour. Whilst his predecessors held that a basic identity existed between Zeus, Nature and Fate, the variable intensity of tonos led Posidonius to believe that they were both separate and related hierarchically. "The first is Zeus, the second, Nature, the third, Fate." Since all differences are material differences, but since Posidonius can speak of Zeus or God as the intelligent governor of the whole even while suffusing every point as the active principle, he seems near to conceiving of a spiritual root-substance from which the differentiated world is derived. Though he may not have done so, the interconnectedness of every aspect of kosmos, the ordered whole, persuaded Posidonius to affirm the value of astrology as a gnostic science and the possibility of divination.

For Posidonius as for other Stoics, the universe is material, but Zeus, who is the binding power of kosmos – that which qualifies matter as an ordered whole – is also its hegemonikon or rational faculty seated in ouranos, the celestial sphere, as the governor of the whole, and in the sun as its inner cohesive impulse. Posidonius thought that the distinction between intelligible and sensible was insufficient to understand the nature of kosmos. Between the realm of pure concepts and the universe of existing things lies ta mathematika, mathematical realities which partake of conceptual and material characteristics. Ta mathematika constitute the World-Soul which gives form to the universe. This radical addition to Stoic physics, drawn from Plato's Timaeus, permitted Posidonius to argue that the World-Soul caused the sphericity of the universe, the geometric shape most conducive to internal motion. He also broke with tradition in teaching that the prota noeta, the Platonic forms, are not merely human concepts but divine ideas which are mediated through ta mathematika. Nature seems in this view to be the body of the world, qualified matter as a whole, and Fate is ananke, necessity, the confluence in a Platonic sense of that which shapes and that which is shaped.

The Stoic school had assimilated human concerns to cosmic concerns, teaching that the essence and structure of the individual human being reflects the essence and structure of the cosmos. Posidonius' conception of physics, however, required a more sophisticated psychology of the individual. For Posidonius, ethics necessarily includes an accurate understanding of the emotions and passions, and unless the source of human wrongdoing and irrationality is located in Zeus, the human soul must contain an irrational component. Either evil is cosmic and fate is unalterable, or evil can be found in the dynamics of the psyche – and the human soul reflects but does not exactly parallel the World-Soul. Posidonius noticed that children and animals are capable of being angry without reason, and this observation led him to accept the Platonic conception of the soul as tripartite. For him the soul had three dynamic movements – reason, a spirited motion and irrational passion. And with Plato he recognized a fundamental distinction between reason and the irrational aspects of soul.

Each part has its oikeiosis, self-appropriation or self-love, but the three parts are ultimately subordinated to two daimones in the soul, one of which impels to good, the other towards evil. The good daimon is the psychological analogue of the World-Soul, but the evil daimon has no analogue unless it is the absence of mathematical form. The good daimon is amenable to reason and logic, but the other can only be compelled to obey. Thus Posidonius objected to Plato's wish to affix preambles to his laws on the ground that the lower daimon, being irrational, is not persuaded by reason and can only be made to obey. Moral evil, therefore, arises from within the soul through predominance of an inferior part: it cannot be eliminated once and for all, but it can be controlled.

The conflict between good and evil, which is the conflict between rational and irrational, kosmos and chaos, is not the result of the soul's encounter with the world, but exists in the individual soul itself. The battle-line in the soul is the emotions and passions, and this is why ethics presupposes psychology.

From the right understanding of the emotions, there may be derived the right understanding of things good and evil, and of the virtues and aim of life.

If the indisputable goal of life is happiness, such an understanding will permit grasping that goal in a comprehensive and philosophical sense. The root cause of unhappiness is the human failure to follow the noetic daimon in the soul and the almost willing weakness involved in succumbing to the lower, irrational daimon. When one attends to the higher daimon, one participates in the potentiality of the World-Soul, whilst acquiescence to the infernal daimon works against kosmos, however futile the effort may be.

Posidonius was dissatisfied with the traditional view that made the individual soul the simple mirror of the World-Soul, because he saw in the effort to follow the superior daimon the possibility of participating in its activity. For Posidonius, the wise man, and even the individual who is making moral progress, shares with the World-Soul the task of continuously creating and sustaining kosmos. The happy human being does not merely live in harmony with Nature; he is one of the creators of Nature. Thus Posidonius gave a fresh and powerful dimension to Stoic ethics.

Whilst some Stoics condemned wealth and the acquisitions of goods as inherently evil, just as they shunned political and social life as an irresistible invitation to moral corruption, Posidonius held that these things were neither good nor evil in themselves. As an active participant in the organization of kosmos, the individual should not fear either material goods or the life of the polis. Rather, he should be vigilant in the use to which he puts his resources and energies. Simple withdrawal from the world is too passive for the Posidonian man: correct participation in all things is the cosmopolitan ideal which signals inner withdrawal from the inferior daimon in the soul. It is not enough, then, to recognize wise men and immoral men – one must know the soul-characteristics of the wise man so that one can through emulation of them make moral progress.

By separating Fate from Zeus, Posidonius firmly renounced rigid determinism while being fully aware of the forces at work in the world. Moral progress is possible because man, as a participant in the divine and informing governance of kosmos, can work to adjust, over the long term, Fate itself. Although kosmos cannot be understood in terms of some fantastic eschatology, moral progress (and therefore a movement towards authentic philosophical happiness) is possible because the organization of kosmos is capable of degrees of improvement. Change occurs as man learns to participate in cosmic activity – becoming cosmopolitan in a transcendental sense – by turning from the lower and embracing the rational daimon in himself. He achieves this through the selfless and assiduous performance of his duties.

Although the surviving fragments of Posidonius' extensive writings do not permit a firm judgement, it seems that Posidonius found the idea of personal immortality absurd. Since the evil daimon is innate to the individual soul and absent from the World-Soul, the soul cannot become immortal by freeing itself of wrongdoing, for the individual soul as a whole cannot join the World-Soul. But the individual can through right ethics place the superior daimon in charge of all aspects of soul, subordinate the passional tendencies by withdrawing from the lower daimon, and thus engage in a microcosmic version of ceaselessly reducing chaos to kosmos. By assimilating the action of the higher daimonic aspect of soul to the celestial motion of the World-Soul, the individual merges the immortal part of the soul with the governing principle of the universe. The wise man does not seek immortality, for he knows there is that in him which is immortal. Perhaps Posidonius found in this standpoint the possibility of self-conscious, though not personal, immortality through cultivation of the highest in the individual.

For Posidonius, ouranos, heaven, offers the paradigm for man. The stars teach ethics. The individual who pursues his duties without emotional involvement in them and without the correlative expectation of results, who recognizes honesty as the good and the hallmark of the wise man, and who seeks to honour the higher daimon in himself discovers a fidelity within the soul which is both its overarching oikeiosis and its link to the World-Soul. He sees that the principles of physics can be translated into the laws of psychology from which are derived ethics and the rules of right conduct. Without wavering in his loyalty to the deepest insights of the Stoic tradition, Posidonius exemplified in his own life and thought the ability of the philosopher to penetrate afresh and more precisely the mystery of the kosmos and the less ordered realm in which human beings dwell. His fearlessness of method and the marriage of observation and abstract thought influenced the generations which came immediately after him, and inspired a number of thinkers in the dawn of the European Enlightenment.