Two gates of sleep; the one of horn, the one of ivory.
Odyssey XIX, 560-565
Undermined, toppled, then half washed away
By memory’s undertow,
The gates of sleep are wrack by day;
Dreams true or false to miscellany go.
Image: Rubble by Flickr user Jennifer Lantigua (Creative Commons license: CC BY-NC-ND 2.0).
In the Odyssey, Penelope speaks of two gates of sleep, one of horn and one of ivory, through which dreams come and go: true dreams issue through the gate of horn (in Homer it is κέρας, “horn”, which puns on κραίνω, “fulfill”); false dreams, through the gate of ivory (ἐλέφας, “ivory”, is like ἐλεφαίρομαι, “deceive”). The figure is common enough in English; I first encountered it in a story by James Tiptree, Jr. (the pseudonymous Alice Sheldon).
As for dreams, they are not the same thing as aspirations, hopes, wishes. C.S. Lewis got it more nearly right when he had the sailors on the Dawn Treader flee in horror from the island where dreams come true.