The principle founder of the Stoic movement was Zeno of Citium who opened a school at Athens around 301 B.C.  Central to his teachings was the law of morality being the same as nature. Zeno taught that the universe was organized along a logical reasonable divine plan (logos). For Zeno God and Nature were essentially the same thing. Logos was, thus, both divine and natural law. How well one lived depended on how well one could live according to that plan. The more complete one’s understanding of the plan, the better one’s life. Zeno believed that a philosophical education was necessary for anyone who wanted to live according to the plan, because only a philosopher could exercise the necessary virtue, reason and patience to allow understanding. By mastering passions and emotions, it is possible to overcome the discord of the outside world and find peace within oneself. Stoicism holds that passion distorts truth, and that the pursuit of truth is virtuous. Greek philosophers such as Cleanthes, Chrysippus, and later Roman thinkers such as Cicero, Seneca the Younger, Marcus Aurelius, Cato the Elder, Cato the Younger, and Epictetus are associated with Stoicism. In Cicero's case, it should be emphasized that while he shared many of the moral tenets of Stoicism, he was not a Stoic himself.

Stoicism first appeared in Athens in the Hellenistic period around 301 BCE and was introduced by Zeno of Citium. He taught in the famous stoa poikile (the painted porch) from which his philosophy got its name. Central to his teachings was the law of morality being the same as nature. During its initial phase it was generally seen as a back-to-nature movement critical of superstitions and taboos. The philosophical detachment also encompassed pain and misfortune, good or bad experiences, as well as life or death. Zeno often challenged prohibitions, traditions and customs. Another tenet was the emphasis placed on love for all other beings.

For Stoics there were no accidents, everything that happened happened for a reason, so every person should try to live in harmony with that plan. In order to do so, one must exercise one’s reasoning faculties and determine what one’s particular place was in the plan. Once one determines one’s place then one can decide what the most reasonable behavior should be. So that we will not be tempted away from contemplation of the logos we need to avoid the distraction of emotional activities and do our duty faithfully in our family and community. The Stoics did not seek to extinguish emotions, only to avoid emotional troubles by developing clear judgment and inner calm through diligent practice of logic, reflection, and concentration. Stoicism, with its stress on public and community service, and a strict adherence to duty, became the most popular philosophy with the Romans. A Stoic school was opened at Rome around 140 B.C. by a Stoic philosopher named Panaetius of Rhodes. Panaetius went even further than his predecessors in stressing the importance of political and family duty in achieving harmony with logos, which made Stoicism even more popular with Romans who basically believed that anyway.  

The ideal is to align one's expectations with nature, wanting nothing beyond what is natural and accepting whatever happens as natural. To be natural is to be rational, to be rational is to be virtuous and to be virtuous is to be happy.

According to Epictetus, a man should accept whatever befalls him as coming directly from God; only a fool would resist. A person can only truly be happy by being virtuous; since virtue is always in the grasp of the human will, nothing external is necessary for happiness. One should avoid the illusion that one needs any external aid to achieve the Good Life. If external props were needed to achieve happiness, then God, being good, would provide humans with these needs. The Stoics argued that there are four passions and affections that interfere with achievement of the Good Life. These are pleasure (hêdonê), sorrow (lupê), desire (epithumia), and fear (phobos). All of these passions focus on  external objects to which a Stoic, as a rational person, should be indifferent.

Among the most important of the Stoic philosophers from our perspective was Panaetius of Rhodes. Panaetius was a pupil of Diogenes of Babylon and Antipater of Tarsus, both heads of the Stoic school in Athens, and he succeeded Antipater as scholarch (head of the school) in 129 B.C. Little is known about his life though it is clear that he spent considerable time in Rome, where he became a member of the literary and philosophical  circle of Publius Cornelius Scipio Aemilianus. This literary group had a profound impact on Roman thought and culture in the later Republic.

Panaetius made a number of changes in Stoic doctrines, the most important of which was to place greater emphasis on personal duty and de-emphasizing the stress on philosophical commitment in order to understand and achieve the Good Life. Panaetius maintained that virtue is not sufficient for happiness, since health, some personal resources, and strength are also necessary. He also divided virtues into the contemplative and the practical, which sits uneasily with traditional Stoic intellectualism. Contemplative virtues are, essentially, those virtues that are practiced by the philosopher. Practical virtues include duties that one should perform toward one’s self, family and society (or state). This new Stoicism is called the Middle Stoa, and its emphasis on action and duty, rather than strictly contemplation and philosophical study, made the Stoicism of Panaetius more popular for consumption by the Roman ruling class.

Romans already believed in duty as the prime principle to guide a citizen in achieving the good life, so Panaetius’ brand of Stoicism offered a message that appealed to the Roman ruling class. It combined the Roman value of pietas (duty to family, the gods and the state) with Greek rationalism in a way that Romans could go for. Stoicism was embraced wholeheartedly by by a number of important Romans like Epictetus, the Younger Seneca, and the Roman Emperor Marcus Aurelius, and it had an influence on a lot of other important Roman thinkers including Marcus Tullius Cicero.