Where does the "sexual ethics" of modern Western culture come from, with its monogamous relationships, mutual marital devotion, and marriage as an absolute value? Does it have purely Christian origins or is it related to the bourgeois "Judeo-Christian ethics," so conducive to the order of capitalist production? Does it begin already before Christianity, in the Hellenistic and Roman eras, perhaps inaugurated by Stoic philosophy and the arts of living that characterized the Greek way of life? Was the ancient world as tolerant towards sex as believed? How does Christianity prepare the rupture it will bring to the idea of sex and subjectivity with its problematic of the "flesh" and "desire"? It is certain that the contemporary "ethical" issue concerning sexual behavior cannot be answered without first shedding light on the genealogical problem defined by the above questions. In 1981, Michel Foucault will deliver this series of lectures at the College de France, marking a decisive turning point in his thinking: the arts of living in the Hellenistic era become centers of meaning from which a new perception of subjectivity emerges, while the careful self-processing by oneself will become the focus of a critical problematic. "Subjectivity" will be perceived as a historically determined depth of oneself, a self-experience that defines relationships with the body, with others, and with the world based on the reasons of truth that are pronounced about the subject. The French philosopher will consider this specific historical space - studying a series of texts from Xenophon and the Stoics to Artemidorus and Plutarch - as a privileged field for the development of such a radical problematic, which does not constitute at all a "return to the subject" but a genealogy of Western subjectivity. If in other research by Foucault "truth" concerned madness, illness, and crime, now it concerns what in modern Western culture corresponds to what is called "sexuality".