The Stoic school of philosophy, founded by the Greek philosopher Zeno of Citium, became the most respected philosophy in Rome. Zeno taught that all of humanity formed a brotherhood of equal individuals. Unheard of in Greek and Roman culture, they even extended this equality to women and slaves. In fact, the greatest Stoic, Epictetus, was himself a former slave.
The Stoics also prioritized the control of the mind, which they considered the key to eudaimonia (“well-being”). According to the Stoics, no one can be totally free from negative external circumstances, but we always have, at least in principle, the option to interpret these events as we like. This allowed the Stoic sage to remain unfazed even in the most horrifying circumstances.
The Stoics also believed in the possibility of finding rational, natural laws on which to base society. This concept would become influential during the Enlightenment, when thinkers tried to construct the ideal system of government based not on the whims of the elite but on natural, eternal, and just principles.
The Stoic school of philosophy was founded by a Cypriot merchant named Zeno of Citium (c. 334–262 BC), who survived a shipwreck after which he ended up in Athens. At a bookshop, he heard a man read a text aloud about Socrates, which convinced him to study philosophy. A few years later, he founded his own school in a public space called the “Stoa Poikile” (“Painted Porch”), from which the word “Stoicism” was derived.
Following Aristotle, Zeno believed that eudaimonia (“well-being”) could not be obtained through external means, such as health, wealth, reputation, and pleasure. These elements are certainly helpful in life, yet they do not by themselves make a person good or bad, happy or unhappy. Instead, Zeno claimed, we should guide our life by reason. But what is reasonable to do? Firstly, it is to realize that much of what happens in our lives is outside of our control, as Zeno’s shipwreck demonstrated. Worrying about these events is of little use, since there is nothing we can do about them. The only correct approach here is to accept the universe as it presents itself with equanimity (ataraxia). A person who has mastered this skill, was termed a “Stoic sage.”
But how do we do this? Luckily, Zeno explained, negative events do not by themselves make us unhappy. What does is our interpretation of them. The great Stoic Epictetus claimed this even applied to death itself. If death is inherently negative, then why was the “Stoic sage” Socrates able to face it with such calm? Instead, it is our conception of death that makes us terrified. He explained:
Men are disturbed, not by things, but by the principles and notions which they form concerning things. Death, for instance, is not terrible, else it would have appeared so to Socrates. But the terror consists in our notion of death that it is terrible. 
The good news is, that although we cannot always change our external circumstances, our interpretation of them is, in principle, always “up to us.” Interestingly, this idea today forms the basis of Cognitive Behavioral Therapy, the best-tested psychological intervention for anxiety, depression, and a host of other psychological problems.
Unfortunately, only fragments of Zeno’s writings have survived, so for a more in-depth understanding, we turn to Zeno’s successors. The most influential Stoic after Zeno was the Greek philosopher Epictetus (c. 55–135 AD), who lived in Rome. Epictetus also had experienced misfortune firsthand, as he had been a slave earlier in his life (his name actually means “acquired”). His sadistic master even twisted his leg until it broke, just for fun. Epictetus’s manual on how to live a good life, known as the Enchiridion (“handy [book]”), starts as follows:
Some things are within your power, while others are not. Within our power are opinion, motivation, desire, aversion, and, in a word, whatever is of our own doing; not within our power are our body, our property, reputation, office, and, in a word, whatever is not of our own doing. 
As an example, Epictetus commented that the average lyre player is “relaxed when he performs alone, but put him in front of an audience, and it’s a different story, no matter how beautiful his voice or how well he plays the instrument. Why? Because he not only wants to perform well, he wants to be well received—and the latter lies outside his control.”  He also wrote that when Zeno met the king of Macedonia, his mindset allowed him to be more confident than even the king:
It was King Antigonus who was anxious before their meeting. Naturally, he wanted to make a good impression, which was beyond his control. Zeno, for his part, had no wish to please the king; no expert needs validation from an amateur. 
Having the control of mind of a Stoic sage is not easy, but without it, we are mere slaves to our circumstances and our emotions, which in turn leads us to make bad decisions and experience misery. In a fragment attributed to Epictetus, he made the same point:
No man is free who is not a master of himself. 
Epictetus paradoxically believed that even a slave or a prisoner could be free in this regard. Similarly, the Greek Stoic Dio Chrysostom (c. 40–115) wrote that even a great king, such as Xerxes of Persia, could inwardly be a slave to his ignorance and passions, while someone who appears outwardly enslaved, such as Diogenes the Cynic (c. 410–323 BC), might nevertheless be inwardly free.
The Stoic philosopher Seneca (c. 4 BC–65 AD) also had his share of bad luck. He was exiled to Corsica after being accused (likely unjustly) of adultery with the sister of Emperor Caligula. During his exile, he tried to comfort his mother in a letter, stating that “the wise man is neither elated by prosperity nor depressed by adversity.” He then added that his predicament wasn’t fully negative either:
These days are my best, because my mind is relieved from all pressure of business and is at leisure to attend to its own affairs, and at one time amuses itself with lighter studies, at another eagerly presses its inquiries into its own nature and that of the universe. 
Emperor Claudius allowed Seneca to return home, and he eventually became advisor to Emperor Nero. When Nero was still young, Seneca could keep the crazy emperor in check, but as he aged, Nero went on a killing spree. When a conspiracy came to light to overthrow the emperor, Seneca (who likely knew about the conspiracy, but did not participate) was ordered to commit suicide. We are told that he stoically put his last affairs in order and then calmy slit his wrists. Unfortunately for him, he didn’t lose his blood quickly enough. To hasten his death, he took poison, but this did not prove fatal either. He was then carried into a warm bath to speed up the blood flow, from which he finally died.
Seneca’s writings mostly focus on self-improvement. For instance:
The spirit ought to be brought up for examination daily. […] What bad habit of yours have you cured today? What vice have you checked? In what respect are you better? 
Associate with those who will make a better man of you. Welcome those whom you yourself can improve. The process is mutual; for men learn while they teach.
He also wrote letters to aid friends dealing with the loss of a loved one. To Marcia, who had lost her son three years prior, he gently stated that it was time for her to resume her life. Grief is natural, but after a while, we must not let it destroy our lives, if only for our remaining friends and family. In a letter to Polybius, who lost a brother, he reminded him that even empires come and go and that rather than despairing, we should rejoice that we were fortunate enough to have known our loved ones, even if only for a short while.
Another influential Stoic philosopher was Emperor Marcus Aurelius (121–180 AD). In his famous work, The Meditations, originally called Addresses to One’s Self, Marcus gave himself advice on how to best live his life (we often read phrases such as “keep in mind” and “do not forget”). The work was likely never intended for publication, but instead served as a self-help manual to keep him on the right path amidst the pressures of the court and during his military campaigns against Germanic tribes:
No retreat offers someone more quiet and relaxation than that into his mind. […] Constantly give yourself this retreat, and renew yourself, [so that you can] wash away all your pain and send you back free of resentment at what you must rejoin. 
In line with his humble and appreciative nature, he began his book with a long list of all the people he was grateful for, including his family members, teachers, and friends:
From my grandfather Verus I learned good morals and the government of my temper. […] From what they say and I remember of my natural father: integrity and manliness. […] From my mother abstinence, not only from evil deeds, but even from evil thoughts; and further, simplicity in my way of living, far removed from the habits of the rich. 
From his teachers, he had learned not to get upset when others did wrong or offended him—and even to “tolerate ignorant people.” This stemmed from his Stoic belief that all humans are imperfect (“the infallible man does not exist”)—and this included himself. To remind himself of this lesson, he regularly reminded himself of his own flaws and was hard on himself to continuously improve:
No more roundabout discussion of what makes a good man. Be one!
To what use, then, am I now putting my soul? Ask yourself this question on every occasion. Examine yourself.
As an exemplary leader, he aimed to be guided not by selfishness, but worked instead to better himself in service to the world. Unique for a man of such power, he also made great effort not to be sucked in by vanity and greed:
Take care not to be Caesarified or dyed in purple. […] Keep yourself simple, good, pure, serious, unpretentious, a friend of justice, god-fearing, kind, full of affection, strong for your proper work. Strive hard to remain the same man that philosophy wished to make you. Revere the gods, look after men. Life is short.
A trick he used to maintain his sanity in the world of the court was to realize the insignificance of each human life. On many occasions, he reminded himself that the “great men” of the past were all dead. Consider the court of Emperor Augustus. His “wife, daughter, grandsons, step-sons, sister, Agrippa, relatives, household, friends, […] doctors, diviners: an entire court dead.” Similarly, he realized “how many a Chrysippus, a Socrates, an Epictetus eternity has already swallowed!” 
Marcus Aurelius also managed to articulate the main tenets of Stoicism in memorable ways. He agreed with his Stoic predecessors that despite the unpredictability of external events, it is up to us how we interpret them. Or in his own words:
Things of themselves have no inherent power to form our judgements.
Or, more famously:
All is as thinking makes it so—and you control your thinking. So remove your judgments whenever you wish and then there is calm. 
This translation is purposefully (and perhaps deceptively) made to resemble a phrase from Shakespeare’s Hamlet (c. 1600):
There is nothing either good or bad, but thinking makes it so.
These judgments, in turn, determine who we turn out to be:
Your mind will take on the character of your most frequent thoughts. Souls are dyed by thoughts.
Reminiscent of Buddhism, Marcus also followed Epictetus’s advice to “not exaggerate [pain] in your imagination,” but instead to stick with the facts:
Do not elaborate to yourself beyond what your initial impressions report. You have been told that so-and-so is maligning you. That is the report: you have not been told that you are harmed. I see that my little boy is ill. That is what I see: I do not see that he is in danger. So always stay like this within your first impressions and do not add conclusions from your own thoughts—and then that is all. 
Fig. 341 – Roman copy of a Greek bust of Zeno (3rd century BD) (Paolo Monti, CC BY-SA 4.0; Biblioteca Europea di Informazione e Cultura, Italy)
Fig. 342 – Marcus Aurelius (c. 170 AD) (Pierre-Selim, CC BY-SA 3.0; Musée Saint-Raymond, France)
Fig. 343 – Roman copy of a Greek bust of Epicurus (c. 200 BC) (Marie-Lan Nguyen, CC BY-SA 2.0; British Museum)
The Stoics also developed the idea that human beings have free will. As we have seen in the Iliad (c. 750 BC), the early Greeks didn’t feel fully autonomous authors of their own destiny but instead felt controlled by both the gods and fate. Although the Archaic Greeks obviously knew concepts such as choice, deliberation, and responsibility, the concept of free will was still nonexistent. Even Aristotle (384–322 BC) didn’t recognize the concept. In fact, according to some scholars, Aristotle didn’t even recognize the will—the faculty that makes us decide to act on our desires or not. According to Aristotle, reason alone was sufficient to motivate us to do something. Following Socrates, Aristotle believed that reason itself desired “the good,” directly causing us to act accordingly. For individuals who have not sufficiently cultivated reason, the appetites—such as hunger and anger—might take over to motivate our actions. Nowhere, however, does Aristotle discuss our ability to decide between following either our reason or our appetites. He always talks as though reason and appetite alone are enough to compel us to make a decision. This curious way of thinking can also be spotted in his definition of the word “choice.” According to Aristotle, a person who acts on appetite instead of reason “acts against his own choice.” Today we would say that this person “chooses to act against reason” or “chooses to act on his appetites.” 
The first steps toward the notion of free will were made by the Athenian philosopher Epicurus (341–270 BC). As a follower of Democritus, he assumed that the world was made out of atoms that moved through the void. He also added that, on occasion, these atoms could “swerve from their paths without cause.” This was necessary to avoid the idea that everything in the world was predetermined. If this were true, nothing that we do would, in any substantial sense, be “up to us.” In reality, he claimed, some things happen “by necessity,” some “by chance,” and some “by our own agency.”
Epictetus (c. 55–135 AD) further expanded on this idea by introducing the will (“prohairesis”), which he defined as the possibility to mentally accept or reject any thought. The will is our inner voice that helps us deliberate what decisions to make. For instance, seeing food when we are hungry is by itself not enough to make us eat. A civilized human has the capacity to not let these feelings be sufficient to impel us to eat. On top of the feeling, the decision to eat requires assent by reason. According to Epictetus, things are “up to us” precisely if our assent was given. And since no one can climb inside our mind, we always have—in principle—the freedom to make these decisions, no matter the circumstances of our lives. It is the only freedom we can always count on, as “no thief can steal your will.”
Paradoxically, the Stoics also believed in a fully deterministic universe. They believed that the universe was an organism endowed with reason (logos), which they equated with God or Nature. As this God is governed solely by reason, everything that happens is predetermined. But how can something be truly “up to us” when life is predetermined? The Stoics solved this problem by assuming that a perfectly wise individual would do on his own accord exactly what the rational god had predetermined for him, simply because they both follow reason. In this way, the will of a wise man is free as it coincides with the predetermined will of God and, therefore, does not require any divine coercion.
Around 200 AD, Alexander of Aphrodisias developed the notion of free will that we are familiar with today. According to Alexander, an action is made freely if the actor could have also chosen not to perform it under identical circumstances. This definition, however, introduces a weakness in the concept that has pushed many modern philosophers to drop the concept altogether. If it is possible to make a pure choice, one not determined by our circumstances or our past, this would mean that the world is not deterministic, which is in conflict with our scientific worldview. This unscientific view, however, soon became one of the great superstitions of the West and also one of its driving forces. The belief that man can take full credit for his own deeds and creations, and thus create a true legacy, can be a powerful motivator to achieve great things and advance society. According to Plotinus (c. 204–270 AD), it also helped fight the fear that we ultimately have no control whatsoever over our lives. Plotinus called it the fear “that we might be nothing.” 
During the time of the Roman Empire, Christians adopted the Stoic notion of free will and used it to rethink the Bible. The Christian theologian Origen (184–253 AD) was highly influenced by the concept but noticed that the Bible made no direct reference to it. He found that the concept could only be inferred from certain passages by assuming God would not punish or reward us for our actions without giving us the option to freely follow his commands. Being impressed by this new reading of the Bible, Origen came to see free will as one of the most important postulates of the Christian faith (in fact, he was the first to attempt to systematically define the axioms of Christianity in his On the First Principles). In his view, God was omniscient, and therefore knew in advance how life would turn out for every person, yet this future was not determined by him, as he had given man free will.
Due to the adoption of free will by Christianity, the concept gained almost universal acceptance in the West. It was formally accepted by the church in the fifth century AD.
Since the Stoics made freedom one of their core values, it was a natural step to want to make it available to all people. The Stoics saw humanity as a brotherhood with each member being endowed with divine reason. As a result, they believed in the equality of all human beings, including women and slaves. Zeno described this view in his Republic, a work that has since been lost.
The Stoic school was unusual in that it accepted anyone. Zeno was a foreigner, Epictetus was a slave, and Plutarch mentioned a female Stoic named Porcia Catonis (c. 70–43 BC). The satirist Lucian (born c. 125) described this Stoic ideal as follows:
They include numbers of barbarians, slaves, cripples, dwarfs and poor; in fact any one is admitted; for their law does not associate the franchise with income, with shape, size or beauty, with old or brilliant ancestry; these things are not considered at all; anyone who would be a citizen needs only understanding, zeal for the right, energy, perseverance, fortitude and resolution in facing all the trials of the road; whoever proves his possession of these by persisting till he reaches the city is ipso facto a full citizen, regardless of his antecedents. Such distinctions as superior and inferior, noble and common, bond and free, simply do not exist there, even in name. 
Slavery had been an accepted fact in most of the world. Just as some people were born into nobility, others were born into slavery. Even the great philosopher Aristotle, who ironically founded the field of ethics, had himself 13 slaves and wrote that “master and slave have nothing in common, as a slave is a living instrument.” Slaves were also commonly regarded as property or animals. For instance, when Galen advised against taking “material losses” too seriously, he added: “If any of my cows, horses, or slaves die, it is not enough to make me grieve.” In some cases, they barely existed in the eyes of their masters. The great poet Horace (65–8 BC), for instance, claimed he had a “habit of walking by himself,” yet only a few sentences later, we find he was actually joined by one of his slaves. 
For the Stoics this was different. Epictetus reminded his wealthy students that their slaves should be viewed as brothers as they were all children of Zeus. Diogenes Laertius (third century), a biographer of Greek philosophers, wrote that some early Stoics condemned slavery as “evil,” and Dio Chrysostom concluded that slavery “had no validity at all” and constituted “unjust servitude.” These were very unusual opinions at the time. Even the Christians accepted slavery as a given.
The Stoic philosopher Seneca, although he owned slaves himself, strongly advocated humanizing them. He asked his readers to “kindly remember that he whom you call your slave sprang from the same stock, is smiled upon by the same sky, and on equal terms with yourself breathes, lives, and dies.” We also read:
I am glad to learn [...] that you live on friendly terms with your slaves. This befits a sensible and well-educated man like yourself. “They are slaves,” people declare. Nay, rather they are men. 
He also wrote that the human soul of a good person is equally divine in both a Roman knight and a slave:
What else could you call [a good] soul than a god dwelling as a guest in a human body? A soul like this may descend into a Roman knight just as well as into a freedman’s son or a slave. For what is a Roman knight, or a freedman’s son or a slave? They are mere titles, born of ambition or of wrong. One may leap to heaven from the very slums. 
Marcus Aurelius tried to implement these Stoic ideals in the real world. In The Meditations, he wrote that the ideal state was “a monarchy which values above all things the freedom of the subject.”  True to his word, he actively worked against slavery by instating new laws. He granted thousands of slaves their freedom in exchange for joining the army, made it easier for slaves to purchase their freedom, tried to convince his people that a person taken by war was not the property of the captor, and encouraged slave owners to bring their slaves to the courts in case of a dispute instead of handling the punishment themselves (although he did not attempt to abolish slavery itself, for which he would find no political support, and he upheld harsh laws for runaway slaves).
The Stoics also made advances towards gender equality. The women of Athens had little freedom. The ideal noble woman, it was believed, was not even supposed to go out in public. Women were also barred from political participation, were not permitted to represent themselves in court, and were not allowed to conduct large economic transactions. In marriage, women were regarded as property under the control of their husbands. Marriages were also generally not very affectionate. This was partly due to the age difference (men generally married at 30, while women married at 14), but also because marriages were usually arranged for political and economic purposes instead of romantic ones.
Surprisingly, foreign women in Athens, such as Aspasia (c. 470–400 BC), the partner of Pericles, were less bound by these traditional constraints. She took this opportunity to participate in public life and became noted for her ability as a conversationalist and adviser. According to Plutarch, her house became an intellectual center in Athens, attracting the most prominent writers and thinkers, including the philosopher Socrates.
In Rome, women generally enjoyed more freedom. They could serve in local politics, could own land, and women could also participate in education. In the higher classes, some women also took part in discussions, and according to Livy, some women even came to the Roman Forum to stand up for their rights. 
Some Stoics aimed at true equality. Especially Musonius Rufus (1st century AD) and Seneca saw women as their equals and encouraged them to study philosophy. The Stoic philosopher Cleanthes (c. 330–230 BC) was of the same opinion, given that one of his works was called On the Thesis that Virtue is the same in Man and in Woman.
The Stoics also set out in search of so-called natural laws, which are laws not based on the whim of a particular ruler but derived from nature by applying reason. These laws, they claimed, are universal and true for any society. The root of this idea goes way back. Heraclitus (c. 525–475 BC) seems to have been the first to believe in the concept, stating, “All human laws are fed by one divine law.” Among the Greeks, Aristotle gave the clearest definition of the concept:
Particular law is that which each community lays down and applies to its own members: this is partly written and partly unwritten. Universal law is the law of Nature. For there really is, as everyone to some extent divines, a natural justice and injustice that is binding on all men, even on those who have no association or covenant with each other. 
The Stoics expanded on this idea. Since they believed that the universe was governed by reason, and since sages operate in accordance with reason, it follows that sages act in accordance with nature. And since sages make good moral judgments, it follows that a natural justice must exist. The Stoics believed that the implementation of this justice would finally result in a worldwide city-state, which they called a cosmopolis, in which all human beings are brothers. This idea started with Diogenes (although some early sources name Socrates), who when asked where he came from, claimed he was a “citizen of the world” (a “cosmopolitan”).
The philosopher and statesman Cicero was deeply influenced by Stoic thought. His elaborations on the concept of natural law were especially influential through the ages. In his work On Law, we read:
Law in the proper sense is right reason in harmony with nature. It is spread through the whole human community, unchanging and eternal, calling people to their duty by its commands and deterring them from wrong-doing by its prohibitions. [...] This law cannot be countermanded, nor can it be in any way amended, nor can it be totally rescinded. We cannot be exempted from this law by any decree of the Senate or the people; nor do we need anyone else to expound or explain it. There will not be one such law in Rome and another in Athens, one now and another in the future, but all peoples at all times will be embraced by a single and eternal and unchangeable law; and there will be, as it were, one lord and master of us all—the God who is the author, proposer, and interpreter of that law. Whoever refuses to obey it will be turning his back on himself. Because he has denied his nature as a human being, he will face the gravest penalties for this alone, even if he succeeds in avoiding all the other things that are regarded as punishments. 
In the 17th and 18th centuries, Cicero’s work inspired Enlightenment thinkers to devise better types of government, one based not on the whims of the elite but on natural, eternal, and just principles.