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 accepted the view that, for Virgil, what we call reality is not in fact such; that Virgil may have considered the Platonic world of the archetypes to be the real world.
Read more about this topic: Gates Of Horn And Ivory