Axiom of Maria is a mystical precept in alchemy where "one becomes two, two becomes three, and out of the third comes the one as the fourth." It is attributed to 3rd century alchemist Maria Prophetissa.
Psychoanalyst Carl Jung interpreted this concept as a metaphor for the process of total individuation of the self. One is the primal state of unconscious wholeness; two deals with the conflict between psychic opposites; three points to a possible resolution (transcendence) and the fourth is a transformed state of consciousness; relative wholeness and at peace.
Jung used the axiom as a metaphor for the process of individuation. One is unconscious wholeness, two is the conflict of opposites, three is the transcendent function("psychic function that arises from the tension between consciousness and the unconscious and supports their union"), and the fourth as the transformed state of consciousness, as a relatively placid whole. Jung speaks of this as a 4-3-2-1 "progression", in the "Psychology of Transference, CW vol. 16. The Axiom of Maria then is an alchemical analogy of the process individuation from the many to the one.