The Stoa Poikile, or Painted Porch, of classical Athens was an unlikely place for a philosophic school to take root. Unlike the tranquil groves used by Plato and Aristotle, this portico bounded the noisy marketplace, where fishmongers and cobblers plied their trades. But Zeno of Citium, a Cyprian immigrant who began teaching there around 300 B.C., was more concerned with practical matters than his forerunners. While Platonists in the airy Academy debated absolute truth, Zeno’s successors, called Stoics after the place where their sect first met, focused on the daily management of the self, founding a tradition that still...