Panaetius is the hinge of the middle Stoa. Born in Rhodes around 185 BCE, he studied at the Stoa in Athens under Antipater of Tarsus, then moved to Rome, where he attached himself to the circle of Scipio Aemilianus — the cultivated Roman general who, with Laelius, ran what Cicero would later call the Scipionic circle. Through this network, Panaetius made Stoic philosophy intellectually respectable in the city that would carry it for the next two centuries.
His major modifications to the system were practical. The earlier Stoa had insisted on a kind of austere rigor — the sage is happy on the rack, virtue is sufficient for happiness, externals are utterly indifferent. Panaetius softened this. He gave room for preferred indifferents (health, wealth, reputation) to matter more — not to the sage's happiness, but to the practical life of the active citizen. He emphasized kathēkonta — appropriate actions, the practical duties of life — over the perfect katorthōmata of the abstract sage. The result was a Stoicism that a Roman senator could actually use.
He also broke with the orthodox Stoa on cosmology, rejecting (or at least suspending) the doctrine of ekpyrōsis — the periodic cosmic conflagration — in favour of an eternal cosmos (Cicero, De Natura Deorum II.118; Philo, De aeternitate mundi 76). This is the most-cited of his doctrinal deviations.
He also emphasized the four roles doctrine — each person has (1) a universal human nature, (2) an individual character, (3) a station in life given by fortune, and (4) a freely chosen vocation. Right action takes all four into account. This is the framework that Cicero turns into the structure of De Officiis.
His treatise On Appropriate Action (Peri tou kathēkontos) has not survived. But Cicero, writing De Officiis about fifty years later, follows Panaetius's outline so closely that scholars treat the first two books of De Officiis as substantially Panaetius preserved in Latin. That makes Panaetius the most influential ancient writer you may never have read directly.
He returned to Athens as head of the Stoa around 129 BCE and died around 109 BCE. His student Posidonius — polymath, geographer, ethnographer, Stoic — carried the tradition forward from his own school in Rhodes rather than as scholarch in Athens, where the school dwindled after Panaetius's death. Together they constitute the entire surviving middle Stoa.