Hence they appeared in the form of quicksilver or Mercurius, lead or Saturn, gold or aurum non vulgi, salt or sal sapientiae, water or aqua permanens, etc. These substances were arcana, and like them philosophy too was an arcanum.
Jung identifies the alchemical arcana as projections of psychological contents onto matter, establishing arcanum as the foundational category linking unconscious psyche to hidden natural substance.
, The Spirit in Man, Art, and Literature, 1966thesis