The Self .. . is an a priori existent out of which the ego evolves. It is, so to speak, an unconscious prefiguration of the ego. Thus ego and Self have a close structural and dynamic affinity. The term ego-Self axis has been used by Neumann to designate this vital affinity.
Edinger establishes the foundational claim that the ego is ontologically grounded in the Self, their relationship constituting the ‘ego-Self axis’ that structures the entire individuation process.
, Ego and Archetype: Individuation and the Religious Function of the Psyche, 1972thesis