3 definitions
Mercurius duplex (n.) is the double Mercurius who ascends and descends, a spirit of light that is also an uncreated self-begetting serpent; the alchemists cared above all for his dark side.
Mercurius duplex (n.) is the unconscious itself, where nothing is differentiated, yet as a living spirit he must appear in two forms: Sol, the active part, by which alone the passive Luna is seen.
Mercurius duplex (n.) is a psychologem that must be essentially dual, since water and fire, spirit and soul, can only define one thing if it unites their contrary qualities; the alchemists saw his psychic nature.