Pierre Hadot is the modern figure who most fundamentally changed how we read the ancient Stoics. Before Hadot, the standard academic approach to ancient philosophy treated it as a series of doctrines — physics, logic, ethics — to be systematized and argued with. Hadot insisted that this approach misses what the ancient philosophers thought they were doing.
In his foundational 1977 essay "Exercices spirituels" and the 1981 collection it gave its name to (Exercices spirituels et philosophie antique), and the books that grew out of them, Hadot argued that ancient philosophy was primarily a bios — a way of life — and that its theoretical apparatus existed in service of practical spiritual exercises (exercices spirituels) intended to transform the practitioner. The view from above, the premeditation of adversity, the prosoche of continuous attention, the dichotomy of control — none of these are doctrines to be debated. They are exercises to be done. And the philosopher is not a person with the right views; the philosopher is a person who is becoming a different person through daily practice.
This reframe has been enormously influential, including outside academic philosophy. Foucault's late lectures on the care of the self descend directly from Hadot. The contemporary revival of practical Stoicism — Robertson, Pigliucci, Holiday — is downstream of him. He gave the tradition back its purpose: not a body of arguments to be read, but a practice to be entered.
The Inner Citadel is his sustained reading of Marcus Aurelius. The book's organizing claim is that the Meditations are structured around Epictetus's three disciplines — the discipline of assent (logic), the discipline of desire (physics, in its Stoic ethical sense), and the discipline of action (ethics). Hadot argues that nearly every passage in the Meditations can be located in one of these three disciplines, and that Marcus is writing himself back into the same practice every day for years.
His name appears in this codex more than any other modern figure because his categories are the ones we are using. Spiritual exercises, the three disciplines, the view from above as a defined practice — all are Hadot's framing of ancient material. The Stoics did not have a word for spiritual exercises. Hadot gave them one.