Epictetus was born into slavery in the first century. His master broke his leg — whether deliberately or through indifference, the sources disagree. He could not change a single external fact of his life. So he turned inward and developed a philosophy of such precision that it became the foundation of modern cognitive behavioural therapy, twenty centuries before the therapists arrived. His handbook, the Enchiridion, opens with the sentence that organises everything in The Quiet Mind: "Some things are in our control and others not."
Marcus Aurelius became emperor of Rome at forty and spent the next nineteen years being dismantled by the role. A plague killed millions across the empire. The northern frontier broke open. He outlived most of his children. Every morning, before anyone else was awake, he wrote to himself in a private journal — in Greek, in a small hand, never intending anyone else to read it. Those notes are the Meditations. They are still in print. They are the most intimate record we have of a powerful man trying, each day, to be a decent one.
Seneca accumulated more wealth than almost anyone alive in his era and wrote, in his private letters, that none of it had brought him a single day of genuine rest. His most famous essay, "On the Shortness of Life," is not about death. It is about the hours and years we give away without noticing — to obligations we never chose, to pleasures we never inhabit, to worry about things that have not happened and may never happen. He wrote it in his mid-forties. It has not aged a day.