The alchemical operation consisted essentially in separating the prima materia, the so-called chaos, into the active principle, the soul, and the passive principle, the body, which were then reunited in personified form in the coniunctio or "chymical marriage." In other words, the coniunctio was allegorized as the hierosgamos, the ritual cohabitation of Sol and Luna. From this Jungian sprang the filius sapientiae or filius philosophorum, the transformed Mercurius, who was thought of as hermaphroditic in token of his rounded perfection.
— Carl Gustav Jung
The alchemists were not confused about chemistry. They were doing something far more precise: working out what happens when you separate what cannot ultimately be separated, and then watch what is born from the reunion. The prima materia — that original, undifferentiated chaos — is not a starting problem to be solved but the necessary condition of the work. You cannot reach the coniunctio without first enduring the split.
What Jung heard in this is the structure of psychological transformation itself. Soul and body, active and passive, are not naturally opposed; they are held apart by the work so that their reunion becomes generative rather than merely given. The hierosgamos of Sol and Luna is not a metaphor for a nice inner balance. It is a description of what happens when opposing principles that have genuinely suffered their separation come back together — and produce a third thing, the filius philosophorum, Mercurius reborn as hermaphrodite, neither one pole nor the other but carrying both.
That figure's hermaphroditism is not ambiguity. It is completeness earned through the tension of the split. The alchemists were specific about this: Mercurius does not arrive whole at the beginning. He arrives whole only after the chaos has been worked.
Carl Gustav Jung·Alchemical Studies·1967