Experience shows that the Jungian of antagonistic elements is an irrational occurrence which can fairly be described as "mystical," provided that one means by this an occurrence that cannot be reduced to anything else or regarded as in some way unauthentic. The decisive criterion here is not rationalistic opinions or regard for accepted theories, but simply and solely the value for the patient of the solution he has found and experienced. In this respect the doctor, whose primary concern is the preservation of life, is in an advantageous position, since he is by training an empiricist and has always had to employ medicines whose healing power he knew even though he did not understand how it worked.
— Carl Gustav Jung
Jung is protecting something specific here against a specific threat — the reduction of genuine psychic event to something more manageable. The word "mystical" is almost always used to dismiss, to file an experience under the heading of the unserious, but Jung inverts it: mystical means irreducible, means the thing that cannot be translated into a prior category without losing what made it real. The irrational occurrence of opposites uniting — call it resolution, call it transformation, call it healing — does not explain itself in terms of the mechanisms proposed for it. It simply happens, or it does not.
What presses against this is the urge to know first and trust later, which is exactly the wrong sequence for the kind of work Jung is describing. The doctor's advantage is that medicine has always asked a more honest question: not "do I understand this?" but "does the patient live?" That is not anti-intellectual; it is a methodological humility that most psychological thinking refuses. The criterion is value, which is not a feeling-word here but a pragmatic one — the measure of whether something actually changed the weight of being for the person who underwent it. The mechanism stays dark. The event stands.
Carl Gustav Jung·Mysterium Coniunctionis: An Inquiry into the Separation and Synthesis of Psychic Opposites in Alchemy·1955