In the Trinity the fourth is God the devil.
Visions : Notes of the seminar given in 1930-1934 (2 Volume Set) (Bollingen)
When you understand analysis as an honest attempt to overcome certain evils of our time, you are not astonished to find that one of the fundamental concepts of the system is the idea of half-divine and half animal libido which is one in itself, and therefore the medicine for an age that is not one in itself, that is suffering from a tremendous dissociation.
Also it is an age when the old triangle values, the Trinity idea, is being reversed, adding the fourth function to the three.
The Trinity consists of God the Father, God the Son, and God the Holy Ghost; and the fourth is God the devil. That makes the square.
You see, this whole astrological picture, this ensemble, is exactly like the tissue of the unconscious, like one of these fantasies, or a dream. Carl Jung, Visions Seminar, Pages 732 – 733.
