Porphyrius (234 – 305), Plotinus’ disciple and editor of his Enneads, addresses his letter to Marcella, an aging woman, whom he married late in life. He explains to her the reasons for leaving her, after only ten months of marriage. He invites her to leave passions behind to lead a philosophical life along the lines of the major ethical principles inspired by Plato. Porphyrius takes a strong stand as an apologist of traditional philosophical teachings. The Letter to Marcella provides a unique account on the ways and principles along which a philosophical life should be led. The Letter to Marcella survives as a single manuscript that dates from the 15th century. It is here introduced, edited, translated and annotated.