The real mystery does not behave mysteriously or secretively; it speaks a secret language, it adumbrates itself by a variety of images which all indicate its true nature. I am not speaking of a secret personally guarded by someone, with a content known to its possessor, but of a mystery, a matter or circumstance which is “secret,” i.e., known only through vague hints but essentially unknown. The real nature of matter was unknown to the alchemist: he knew it only in hints. In seeking to explore it he projected the unconscious into the darkness of matter in order to illuminate it. In order to explain the mystery of matter he projected yet another mystery – his own psychic background -into what was to be explained: Obscurum per obscurius, ignotum per ignotius!
C.G. Jung, Psychology and Alchemy (Part 3, Chapter 2.1)