Among the cardinal virtues of classical Greece—wisdom, courage, justice, piety, and sôphrosunê—sôphrosunê is now the least understood or esteemed. But, as this book shows—studying the vigorous and wide-ranging debates about the virtue across fifth- and fourth-century poetry, prose, and philosophy—many Greeks in fact judged it the preeminent virtue. They understood it to be the capacity to choose between one’s conflicting desires and to act only on those aiming at what one judges one’s authoritative or properly life-defining ends. This is the capacity to be a mature human: facing down one’s bodily and social promptings with discipline, identifying with one’s long-term goals or acknowledged norms over one’s evanescent impulses. This is what makes one count as an “agent”: someone who acts on her own principles rather than on external or internal proddings. Thus sôphrosunê is the virtue of agency. The clearest evidence for this is the disagreement found across ancient Greek literature over the term’s application and scope. This book starts by appraising remarks about sôphrosunê from the archaic and early Classical period in Homer, Theognis, Pindar, Aeschlyus, Heraclitus, and funerary inscriptions. Then it turns to later fifth-century exchanges in Euripides especially but also Herodotus, Thucydides, Aristophanes, Critias, Antiphon, and Democritus. Socrates is a crucial figure for the study, as we see in the works of his associates Antisthenes, Xenophon, and particularly Plato. After several chapters on Plato, we turn to the radical innovations of Aristotle and some less familiar Pythagorean assessments.