This memorial essay on the French historian of philosophy Pierre Hadot (1922–2010) explores his life and work. Starting out from an ecclesiastical background and education, Hadot's interest in mysticism led him to study the late Greek Neoplatonists Plotinus and Porphyry, as well as the Latin Church Fathers. Elected first to the École pratique des hautes études and then to the Collège de France, Hadot developed his most influential idea, that ancient philosophy was not the construction of an abstract system of ideas, but a concrete, lived practice intended to transform the perception and being of the practitioner, and thereby to ensure his or her happiness. Later in his career, Hadot, influenced by German Romanticism and above all by Goethe, turned to study the history of the concept of nature from classical to modern times. The breadth of his interests and the novelty of his approach account for the wide interest in his works, extending to circles that extend far beyond the disciplines of classical studies and the history of philosophy.