The article examines the metamorphosis of the Platonic Principle of Good in the doctrine of Basilides, the 2 nd century Christian gnostic. The Basilidian doctrine represents a radical form of dualism, in which the universe-physical and metaphysical-is born from an encounter between Light (good) and Darkness (evil). In his effort to liberate the Light from all contact with Darkness, Basilide refers to several different mediators (eye, mirror, gleam, desire). Analysing the Basilidian myth in the light of the Platonic writings brings out the paradoxes inherent in any attempts to construct a dualistic metaphysics within the Platonic context.