Posidonius of Apamea was yet another Stoic born into a prominent family in a time of anxious abundance. The year of his birth, 135 BC, in what is now Syria, marked the beginning of political turmoil that in a sense continues to this day. But for Posidonius, it was the same kind of formative incubation in uncertainty that had created Zeno and Cleanthes before him.

Perhaps these are the ideal conditions in which Stoicism emerges: a homeland lacking strong leadership and buffeted by powerful outside forces; a ringside seat to the perils of excess and greed. It was all an early lesson that in an unpredictable world, the only thing we can really manage is ourselves—and that the space between our ears is the only territory we can conquer in any kind of certain and enduring way.

In any case, Posidonius would later recall with disapproval that the abundance of Syria in those days made its people “free from the bother of the necessities of life, and so were forever meeting for a continual life of feasting and their gymnasia turned into baths.” He wrote of the “drunken ambition” of the local tyrants. Things were good, but good times rarely make for great people, or great governments.

Ultimately, he voted with his feet as many of the other early Stoics did, leaving his birthplace at eighteen or twenty years old for Athens.

When Posidonius arrived in Athens sometime between 117 and 115 BC, he found the Stoa Poikilē firmly in the hands of Panaetius, who was by then an old man and a towering figure, not only in the Stoic school but also in the empire. Fathers from across the Roman ruling class—from senators to generals and even kings of distant provinces—had begun to have their children educated by philosophers. Where Panaetius had taught Rutilius a generation earlier in Rome, many of Rome’s richest and most powerful were now sending their most promising children to him in Athens to prepare them for entry into Roman life.

And yet even amid these star pupils from the Eternal City, the young Posidonius must have stood out for his brilliance. Sources portray him as a polymath with diverse interests in natural history, astronomy, meteorology, oceanography, geography, geology, seismology, ethnography, mathematics, geometry, logic, history, and ethics. Perhaps it was Panaetius, who had traveled widely on his fact-finding mission, who encouraged his young student to take his studies on the road. What we know is that after his time in Athens, the next large chunk of Posidonius’s life was spent studying in far-flung places from Italy, Sicily, Greece, and Dalmatia to North Africa and the Near East.

It cannot be said that the Stoics, like some philosophers, were interested only in their proofs or debates. Posidonius perfectly illustrates the curiosity, the fascination with the beautiful and complex world that surrounds us which defined Stoicism in the ancient world and continues today. You can and should be interested in everything, the Stoics taught, because you can and should learn wisdom from everything. The more you experience, the more you learn, and, paradoxically, the more humbled you are by the endless amounts of knowledge that remain in front of you.

As Posidonius traveled, his reputation grew as the greatest polymath since Aristotle. He measured tides in Spain and conducted ethnographic research on the Celts in Gaul. He was a keen observer and a lover of data—regardless of the discipline—and a diligent recorder of it all. He measured the circumference of the earth, the size and distance of the sun and the moon, created models of both the globe and the known solar system. The only constraint on his brilliance was the crude measuring tools of his age, which often skewed his calculations. Still, his ceaseless travel and unmatched curiosity dramatically increased the understanding of the known universe at that time.

It also got him outside, down in the dirt and out on the water. His classroom was the sky and the stars and the bustling marketplace, as it had been for Zeno, as it is for the child who can be fascinated even by an ordinary patch of grass. Posidonius lived, as Seneca would later write, as if the whole world was a temple of the gods.

Some geniuses are content to live entirely in their own heads. Many philosophies—filled with philosophers who no doubt see themselves as geniuses—subtly encourage this tendency. Epicureanism, for instance, which was resurgent in Posidonius’s time, encouraged its followers to turn away from the world, to ignore politics and the noise around them. Posidonius, thanks to the influence of Stoics like Diogenes and Panaetius, resisted the pull of this bubble and, like a good Stoic, also turned his intellect to politics and governance.

Indeed, his far-flung travels brought him in constant contact with Roman legions, legions that had been trained for Marius by his fellow Stoic and student of Panaetius, Rutilius Rufus. We see evidence from the fragments of his writings that Posidonius studied troop movements, the history of warfare, and the customs of local people, and even gathered intelligence about foreign powers, which he not only provided to generals but later used in his many books. He even wrote a manual on military tactics, a kind of Art of War, that was so detailed that it was considered too advanced for anyone but a general. In addition to becoming a repository of military tactics, he also had deep ethnographic insights gathered from the foreign territories, which generals like Pompey would seek from him for many years.

He was the complete man. An explorer. Strategist. Scientist. Politician. He was, then, a real philosopher.

At some point, though, every traveler must come home, which for Posidonius became Rhodes. Putting his study of politics into practice, he rose in the leadership ranks there to the highest civil position of prytany, presiding over the governing council in Rhodes while building up his philosophical school.

His political duties would bring him to Rome in 86 BC on embassy, but it was likely his curiosity and desire to study human beings that brought him to the deathbed of one of Rome’s worst strongmen, Marius. Marius was elected to his seventh consulship in late 87 BC, and seemed to think that his political power made him immortal. He could not have imagined that Posidonius would be one of the last faces he would ever see.

Delusional, tortured by dark dreams, wearied by a life of endless ambition and the creeping fear that it had all been for naught, Marius received Posidonius, a keen observer, who was repulsed by what he saw. A few days after the meeting, Marius died, convinced until the end that he would once again be leading troops in battle and expanding his conquests. As a Stoic, Posidonius must have noted—as Plutarch would—what a far cry this was from the peaceful passing of a philosopher like Antipater, who had spent his final moments counting the blessings of his voyage through life.

It’s a timeless question: If you actually knew what “success” and “power” looked like—what it did to the people who got it—would you still want it?

Posidonius’s later writings are filled with firsthand observations about the costs of ambition and insatiable appetites. In one of his histories, he writes of a philosopher named Athenion, who had designs on becoming a tyrant in Athens. It must have struck Posidonius how easily people can be corrupted and cut off from virtue, for here was a man with similar training who had abandoned his genius to marry a prostitute and depended on the mob for his own political advancement.

In another account of a revolt in Sicily, he spoke of a Damophilus, “a slave of luxury and malpractice, driven through the country in four-wheeled chariots at the head of horses, luscious attendants, and a concourse of bumsuckers and soldier-slaves.” Almost with a sense of satisfaction, Posidonius tells us how Damophilus came to a violent and painful end at the hands of his slaves. We can imagine Posidonius expecting a similar comeuppance for Apicius, the gluttonous and greedy monster who was responsible for bringing his friend Rutilius Rufus up on a false charge.

What linked all these historical cases together in Posidonius’s mind was a deficiency of character. “Robbers, perverts, killers, and tyrants,” Marcus Aurelius would later write, “gather for your inspection their so-called pleasures!” Posidonius had actually done that, been in the room with Marius, inspected would-be tyrants and killers up close just as he had observed the tides and the movements of the planets.

From this he was able to pass along insights that were just as valuable as his scientific ones: Be wary of ambition. Avoid the mob. Luxury, as much as power, rots. From Seneca, we get Posidonius’s final judgment on Marius: “Marius commanded armies, but ambition commanded Marius.” Seneca paraphrased, “When such men as these were disturbing the world, they were themselves disturbed.”

After Panaetius’s death in 109 BC, Posidonius would leave Athens for the last time, convinced that the people had become simply “a mindless mob” (ochloi anoetoi). Posidonius probably didn’t think much better of the Romans, after what he had seen firsthand.

Rhodes, at once both isolated and still central to the flow of goods and ideas across the Mediterranean, was a perfect perch for this independent thinker. Posidonius worked on his histories and his theory of the human personality during this time, and they both reflect a more realistic and disillusioned appraisal of his fellow man—an assessment often shared by geniuses. But just as this view was settling in, Posidonius was visited by a bright light. In 79 BC, a young Cicero, then twenty-seven years old, and a kind of once-in-a-generation talent (like Posidonius), would make his way to Rhodes to study under the great man. Panaetius had his Posidonius and now Posidonius would have his own brilliant pupil, who in turn would lovingly refer to his teacher in his writings as “our Posidonius.”

The rest of his years would be spent writing and philosophizing, and of course teaching. It’s clear that his travels and his real-world experience in politics at the highest level informed all three of these domains. Posidonius, like his teacher Panaetius before him, held aristocratic views—today we’d call him one of the “elites.” Except unlike today’s elites, who are often out of touch and surrounded by a bubble of their own like minds, Posidonius had formed his suspicions of the mob and populism from firsthand experience.

He had seen the world, he had seen war, which formed a philosophy—grounded in natural sciences, history, and human psychology—that was sought after by the most important people of his time. This is undoubtedly what drew Pompey, the great general just then rising to power, to Rhodes to attend Posidonius’s lectures.

In 66 BC, before his campaign against Cilician pirates, Pompey visited Posidonius and in a private audience asked him if he had any advice for him. Posidonius, quoting Homer, told him to “be the best and always superior to others.” It was subtle moral advice whose meaning Pompey, with what Posidonius would later call “his insane love of a false glory,” ultimately missed.

“Best,” to the Stoics, did not meaning winning battles. Superior did not mean accumulating the most honors. It meant, as it still does today, virtue. It meant excellence not in accomplishing external things—though that was always nice if fate allowed—but excellence in the areas that you controlled: Your thoughts. Your actions. Your choices.

Even so, the glory-seeking general remained a respectful student. At the height of his powers, following his great victories in the East during the Third Mithridatic War, Pompey made a return visit to Posidonius in 62 BC, and bowed with his army standards lowered at the philosopher’s door. Perhaps Pompey did, in his own way, grasp what Posidonius meant by “best,” even if he could not live it.

Despite being stricken by a severe case of arthritis and gout during this visit, Posidonius gave Pompey a private lecture from bed on why only the honorable is good, in which, through cries of pain, he had to stress that he would still not admit that pain was an evil.

This triumph over pain—over oneself—that’s the “best” Posidonius was talking about. More impressive, he did live it.

In his writings, Posidonius held that the mind seeks wisdom and what is truly good, whereas the lower parts of the soul seek power and the glory of victory (like Pompey), as well as bodily pleasure. Good habits and lifestyle—set in place by the mind—are checks against these irrational parts of the soul. This idea that part of us is rational and part of us is not was a fairly radical departure for the Stoics, who had long held that the whole self was rational.

But this inner battle—as Martin Luther King Jr. would later call the Civil War between the “North” and “South” of our souls—rings true to any person with a shred of self-awareness. We have competing parts within us, and what matters in life is which side we choose to turn ourselves over to. One must design one’s life, Posidonius said, “to live contemplating the truth and order of the universe and promoting it as much as possible, being led in no respect by the irrational part of the soul.” This was a feat that Marius and Sulla, Athenion and, sadly, even Pompey could not seem to achieve, despite all their cunning or military might.

That’s because it’s really, really hard to do—whether you’re a genius or a conqueror. If you can do it, though, the Stoics believed, you will produce something far more impressive than brilliant writing or splendid victories.

Earlier Stoics had attempted to divide the philosophy into three parts, using the analogy of a farm or an orchard, with a field (physics), fruit (ethics), and fence (logic). Sextus Empiricus tells us that Posidonius differed: “Since the parts of philosophy are inseparable from each other, yet plants are distinct from fruit and walls are separate from plants, he claimed the simile for philosophy should rather be a living being, where physics is blood and flesh, logic the bones and sinews, and ethics the soul.” It’s the perfect metaphor for the Stoics too, because philosophy is meant to be lived as a human being.

Building from Chrysippus and Zeno, Posidonius took this idea even further. He saw the entire cosmos as a sentient, living being in which all things are interconnected (sympatheia). The study of science can sometimes lead a person to atheism, but in Posidonius’s case his experiments with the tides and his observations of the stars had given him a strong sense of a creator—that there was a providential fate governing the universe. Pushing beyond Chrysippus’s “no shoving” rule, he believed every human being was quite literally on the same team. We are all tied together in cosmic sympathy, Posidonius believed, and none of us are entirely self-sufficient or autonomous. Each of us has been given a role in this large body—one of us is a finger, another a skin cell, another a liver—and we exist in collaboration and tension with each other. It was God, he thought, that ran through this organism as pneuma—a kind of soul of the universe.

In his later years Posidonius dedicated himself almost entirely to completing his great histories. Spanning fifty-two volumes and representing a full third of his entire literary output, his histories picked up at Carthage in 146 BC with Scipio Aemilianus and continued up to Sulla’s sack of Athens in 86 BC. Strabo says that he was even writing a separate work dedicated entirely to Pompey. His known works ranged from those on fate and ethics to others dealing with emotions and the ever-present enemy for the Stoics: anger. He also wrote on grief and duty, and of course many scientific books based on his early explorations on the oceans, weather, and circumnavigation of the earth.

Although only fragments of these great works survive and Posidonius is mostly unknown today, he was a towering figure in his own time and for long after. Centuries later, Saint Augustine, in his famous City of God, took the time to call him out by name and respond to the most scientific of the Stoics, if only to criticize his use of astrology. Posidonius may not be a household name today, but what author wouldn’t have been satisfied to still be cited some five hundred years after his death? And by a saint no less?

Posidonius worked and lived in many places in his long life—Syria, Athens, Rome, and Rhodes—and he traveled almost the entirety of the known world. He wrote many books. He advised many powerful men. He was one of the smartest men of the ancient world—a small part of a cosmic universe, by his own admission, but an impressive contributor nonetheless.

And yet even geniuses are eventually forgotten, and ultimately all of them are mortal. No Stoic would dispute or fight this, Posidonius least of all.

In 51 BC, he died peacefully at age eighty-four, and though we don’t have any record of it, we can imagine him having learned to depart from the world a happier and more grateful man than what he had seen of Marius’s haunting and ignoble end.