In Homer’s Odyssey, we first learn, with Penelope, about the two gates of dream: the Gate of Ivory and the Gate of Horn. Dreams that come through the Gate of Ivory are “dangerous” and may not be manifested; dreams that come through the Gate of Horn are clearer, and may be embodied in events. The difference seems to be a matter of clarity rather than deception. Carved ivory is totally opaque; polished horn is translucent.
Synesius of Cyrene wrote a marvelous treatise on dreams around 405 in which he asserted that it is the weakness of our understanding, not confusion or deception in dreams themselves, that makes some dreams seem false. "The Penelope of Homer assumes that there are two gates of dreams, and makes half of them deceptive dreams, only because she was not instructed in the matter. For if she had been versed in their science, she would have made them all pass out through the gate of horn." Penelope was "guilty of ignorance" about her own power of inner sight, distrusting her dreams without reason. Therefore "we should not confuse the weakness of the interpreter with the nature of the visions themselves".
The Gates of Horn and Ivory reappear in the Aeneid but Virgil changes the characterization in his account of Aeneas’ descent to the underworld to visit his dead father Anchises. Now dreams that come through the Gate of Ivory are designated “false”, while those that come through the Gate of Horn are “true”. This becomes a standard distinction for centuries in the minds of Westerners raised on the classics.
However, there is a mind-trap in Virgil's story. Anchises sends his son back from the Underworld through the Gate of Ivory. Aeneas and the Sybil return to the regular world through the gate of empty dreams. Is the poet hinting that our ordinary experience of reality is the false dream?
Image: Aeneas and the Sybil return to the ordinary world through the Gate of Ivory. Codex Vaticanus Folio 57r