This chapter describes the 'Gnostic school of thought' and the diverse strategies of self-differentiation that it used and elicited. Irenaeus reports that 'Valentinus adapted the fundamental principles of the so-called Gnostic school of thought to his own kind of system' and gives a summary of a myth that the Gnostics taught. The Gnostic myth was a bold attempt to explain the origin and fate of the universe through a combination of the Jewish scriptures and Platonist mythological speculation. Marcion, Valentinus and Justin Martyr developed a set of responses to the Gnostic sect and/or each other that their successors borrowed and developed. These strategies ranged from outright rejection through heresiological rhetoric and withdrawal of fellowship, to adaptation and Christianisation of the Gnostic myth, to more philosophical modes of authority. If the construction of a 'Gnosticism' obscured the characters of the persons and groups assigned to it, likewise the category 'proto-orthodox' can homogenise and so distort the diversity of pre-Constantinian Christianity.