One may construe the Sethian Gnostic picture of the world and its origins as an interpretation of the biblical protology of the book of Genesis in the light of the Platonic distinction between an ideal, exemplary realm of eternal stable being and its more or less deficient earthly and changeable copy, in which the principal Platonic dialogues of reference are the Timaeus and the Parmenides. Various Sethian treatises offer us accounts of the origin and generation of both these realms; while their portrayal of the origin and deployment of the earthly realm is unmistakably influenced by their readings of Plato's Timaeus, their account of the origin and deployment of the ideal realm is noticeably influenced by readings of Plato's Parmenides. This article attempts to show that the shift from the Timaeus as the primary Platonic dialogue of reference for the Middle Platonic thought of the first two centuries to the Parmenides as the primary dialogue of reference for the emerging Neoplatonism of the third century is also visible in the Sethian treatises. In mid- to later second-century Sethian treatises, the cosmology of the Timaeus serves as an exegetical template to interpret the Genesis protology, but with the turn to the third century, the Sethian trestises that circulated in Plotinus' circle have abandoned all interest in the Genesis protology in favor of a theology of transcendental ascent. One may construe the Sethian Gnostic picture of the world and its origins as an interpretation of the biblical protology of the book of Genesis in the light of the Platonic distinction between an ideal, exemplary realm of eternal stable being and its more or less deficient earthly and changeable copy, in which the principal Platonic dialogues of reference are the Timaeus and the Parmenides. Various Sethian treatises offer us accounts of the origin and generation of both these realms; while their portrayal of the origin and deployment of the earthly realm is unmistakably influenced by their readings of Plato's Timaeus, their account of the origin and deployment of the ideal realm is noticeably influenced by readings of Plato's Parmenides. This article attempts to show that the shift from the Timaeus as the primary Platonic dialogue of reference for the Middle Platonic thought of the first two centuries to the Parmenides as the primary dialogue of reference for the emerging Neoplatonism of the third century is also visible in the Sethian treatises. In mid- to later second-century Sethian treatises, the cosmology of the Timaeus serves as an exegetical template to interpret the Genesis protology, but with the turn to the third century, the Sethian trestises that circulated in Plotinus' circle have abandoned all interest in the Genesis protology in favor of a theology of transcendental ascent.