Four great expositors of Stoicism emerged in Rome in the first and second centuries CE: Seneca the Younger, Gaius Musonius Rufus, Epictetus, and Marcus Aurelius—an imperial courtier, a member of the equestrian order, an ex-slave, and an emperor. Despite being men of their time, their voices reach us clearly across the centuries because they speak always to our common humanity and our best aspirations. Since the work of earlier Stoics exists only in fragments, the surviving writings of these men constitute for us by far the most detailed record of Stoic philosophy.

Seneca focused on writing, and his books, plays, and letters were widely disseminated in his lifetime. Musonius Rufus was a popular teacher, and some of his lectures were transcribed by students and published. Marcus wrote for himself, and his private notebooks, known to us as Meditations, did not become widely available until the tenth century. Epictetus focused on teaching; he wrote nothing himself, but some of his lectures were transcribed or summarized, and quickly became well known. Within fifty years of his death, he was praised at length in a rock-cut inscription from Pisidia (southern Turkey), and anecdotes about him were cropping up in other works of literature. Within a hundred years, one writer asserted that Epictetus was more widely read than Plato. The fundamental question that his teaching was designed to address is: How must we live if our lives are to be subjectively fulfilling and objectively worthwhile? And his answer was: Get ever closer to yourself, your true self.

At the heart of his teaching is an easily understood and insightful distinction. Some things are not "up to us"—they are external to us and just happen to us—but our minds are our own, and we can control our internal states and our responses to events: our desires, inclinations, choices, judgments, and intentions—in short, all the things that make up our characters. These things are up to us and cannot be thwarted by anyone or anything else. We need to learn to accept experiences and events over which we have no control, to recognize them as "indifferent" (neither good nor bad, and with no contribution to make to our happiness or misery), and to use them as the materials on which to practice virtue; we need to see them as gifts of the providential God who steers and arranges everything in the universe, great and small. In this way, fated events become opportunities, not constraints.

Epictetus disputing with Hadrian. (1436) Produced at Switzerland, Basel; miniatures by a French artist, script and initials by an Italian scribe. Bodleian Library shelfmark MS. Canon. Misc. 378.

No one can stop us doing the things that are up to us; Epictetus constantly stresses that in this realm we are free: "The things that are up to us are naturally free, unimpeded, and unobstructed, while the things that aren’t up to us are weak, enslaved, subject to impediment, and not ours." Epictetus himself, as already mentioned, was an ex-slave (this is one of the few facts we know about his life), and there is no doubt that he was marked for life by his time as an enslaved person. More than any other ancient philosopher, he insisted that, however else one describes the goal of life, it must be a state of freedom, where "everything that happens to a person is in accord with his will and no one is able to impede him." Slavery most frequently occurs in the discourses in this metaphorical sense, but Epictetus reflected on literal slavery as well. He claims at one point that the institution of slavery is not natural, but a matter of human convention; by nature, all human beings, whatever their status, are equal just because they all possess reason. Despite this, however, Epictetus seems to accept slavery as an institution. If he was no active abolitionist, that was presumably because, for a Stoic, one’s station in life is a matter of little or no relevance to one’s moral progress or ultimate happiness.

If we understand that our happiness does not depend on our bodies, possessions, and careers but on the mental faculties that are up to us, we will free ourselves of negative emotions (those that are not supportive of our natural potential for goodness), and our rational faculty, the distinctive part of a human being, will begin to work better, now that it is uncluttered by unnecessary reactions. It will begin to judge things correctly, and since everything bad that happens to us is the result of our own wrong judgments, we will start to live well. It is a fact of our human nature that we have all the resources we need to achieve happiness and a smoothly flowing life. No one empowers or disempowers us except ourselves. Moment by moment, we need to assess our mental condition to make sure that we are focusing only on things that are up to us and not letting externals worry us. That way, we can take control of our lives.

This focus on what is up to us increasingly aligns us with the Reason that guides the universe, which is God. For the human soul, in its rational aspect, is a fragment of God. Rationality is our birthright and an inalienable human capacity. Once our rational faculty is working well, we can act freely as independent and responsible moral agents, perpetuating the divine plan for the world by always acting virtuously. And virtue, which is the proper functioning of the soul in accord with its nature under the guidance of reason, always fulfills and benefits us and those around us, so that virtue is the essential prerequisite of happiness and peace of mind. If we are virtuous, nothing external to our minds can make the slightest difference to our happiness—not poverty or ill health, or even the imminent prospect of violent death. We therefore need to safeguard what is ours, our mental faculties, and turn away from everything else.

Since it is our nature’s goal to be rational and to act virtuously, it is the job of philosophy to help us realize this potential by curing us of attachment to everything about us that is not conducive to this end. By relying on reason and focusing only on what is up to us, we will be fulfilling our nature as human beings, which is happiness, and there is no one in the world who does not want to be happy. It will take hard work and total commitment, and no one should undertake the work unless they are prepared to change. A teacher such as Epictetus can show the way, but he cannot do the work for us. Theory, information, and advice from others are useless unless they are acted upon and put into practice by the student.