Gates of horn and ivory

The gates of horn and ivory are a literary image used to distinguish true dreams (corresponding to factual occurrences) from false. The phrase originated in the Greek language, in which the word for "horn" is similar to that for "fulfil" and the word for "ivory" is similar to that for "deceive". On the basis of that play on words, true dreams are spoken of as coming through the gates of horn, false dreams as coming through those of ivory.

The Odyssey
The earliest appearance of the image is in the Odyssey, book 19, lines 560-569. There Penelope, who has had a dream that seems to signify that her husband Odysseus is about to return, expresses by a play on words her conviction that the dream is false. She says:
 * Stranger, dreams verily are baffling and unclear of meaning, and in no wise do they find fulfilment in all things for men. For two are the gates of shadowy dreams, and one is fashioned of horn and one of ivory. Those dreams that pass through the gate of sawn ivory deceive men, bringing words that find no fulfilment. But those that come forth through the gate of polished horn bring true issues to pass, when any mortal sees them. But in my case it was not from thence, methinks, that my strange dream came.

The translator of the Loeb Classical Library edition of the Odyssey comments:
 * The play upon the words κέρας, "horn", and κραίνω, "fulfil", and upon ἐλέφας, "ivory", and ἐλεφαίρομαι, "deceive", cannot be preserved in English.

An echo in later Greek literature
A reference to the Odyssean image appears in the late (c. AD 400) epic poet Nonnus:
 * As Morrheus slept, the vision of a dream cajoled him,
 * beguiling his mind after flitting through the gates of ivory.

The Aeneid
Virgil borrowed the image of the two gates in lines 893-898 of Book 6 of his Aeneid, describing that of horn as the passageway for true shadows and that of ivory as that through which the Manes in the underworld send false dreams up to the living. Through the latter gate Virgil makes his hero Aeneas, accompanied by the Cumaean Sibyl, return from his visit to the underworld, where he has met, among others, his dead father Anchises: 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. Of various things discoursing as he pass'd, Anchises hither bends his steps at last. Then, thro' the gate of iv'ry, he dismiss'd His valiant offspring and divining guest.

Why Virgil has Aeneas return through the ivory gate (whence pass deluding lies) and not through that of horn is uncertain. One theory is that it refers to the time of night at which he returned. Jorge Luis Borges suggested that Virgil meant that what we call reality is not in fact such.

Other Latin writing
In his Silvae V iii 285-290, a lament for his dead father, the poet Publius Papinius Statius, expresses the wish that his father may come to him from the abode of the dead in the form of a true dream, passing therefore through the gate of horn: Thence mayst thou pass to where the better gate of horn o'ercomes the envious ivory, and in the semblance of a dream teach me what thou wert wont to teach.

English writing
The gates of horn and ivory appear in the following notable English written works:
 * Edmund Spenser's epic poem "The Faery Queene" (1590, English) in book 1, stanzas XL and XLIV, in reference to a false dream being brought to the hero (Prince Arthur/the Knight of the Red Crosse).
 * E. M. Forster's short story The Other Side of the Hedge. The reference from Forster comes when the main character of the story observes the two gates; The Other Side of the Hedge is usually read as a metaphor of death and Heaven.
 * T.S. Eliot's poem "Sweeney Among the Nightingales," the line "And Sweeney guards the horned gate" is likewise a reference to this image.
 * Eliot's poem Ash-Wednesday. The lines "And the blind eye creates / The empty forms between the ivory gates" similarly refer to this concept.
 * H. P. Lovecraft's story, "The Doom that Came to Sarnath," as a set of magnificent ivory gates, carved from one piece of ivory stood at the entrance of a city of vain humans, which seems to be taken from Lord Dunsany's story "The Idle Days on the Yann". It is also mentioned as a passage to the realm of hallucinations in Lovecraft's "Celephaïs."
 * Ursula K. Le Guin's novel A Wizard of Earthsea
 * Neil Gaiman's comic book series The Sandman
 * Robert Holdstock's novel Gate of Ivory, Gate of Horn. In the Holdstock novel, the main character grapples with a traumatic event that has two very different manifestations, one true and one false.
 * Derek Mahon's poem "Homage to Malcolm Lowry" by Derek Mahon. "Lighting-blind, you, tempest-torn / At the poles of our condition, did not confuse / The Gates of Ivory with the Gates of Horn."