The Gates of Horn and Ivory are the dual passages through which dreams enter the world of the living. The huge preponderance of dreams pass through the Gate of Ivory-- confusing, meaningless, usually forgotten. But the rare dream that passes through the Gate of Horn is sure to have deep significance for the dreamer, and is forgotten, ignored or uninterpreted at the dreamer’s peril. These are the dreams that convince us that there are vast universes beyond us, a collective unconscious, separated and separating by the thinnest of filaments.
Two gates the silent house of Sleep adorn;
Of polish'd ivory this, that of transparent horn:
True visions thro' transparent horn arise;
Thro' polish'd ivory pass deluding lies.
-- Virgil, Aeneid (tr. Dryden)