According to Tolkien, some fairies serve the semi-Platonic function of acting as “agents” in impressing upon creation the divine ideas of the natural world. Possibly related to this is the description in The Silmarillion of the withdrawal of Valinor from the circles of the Arda following the rebellion and destruction of Númenor:
“For Ilúvatar cast back the Great Seas west of Middle-earth, and the Empty Lands east of it, and new lands and new seas were made; and the world was diminished, for Valinor and Eressëa were taken from it into the realm of hidden things.” (Silmarillion 279)
Rather than “Platonizing fairy-land,” perhaps it would be more accurate to say that Tolkien “Fairy-izes Plato”: the undying, transcendent “realm of hidden things” that men long for and which infuses our own world with its sense of beauty and wonder, is not some putative, unchanging “intelligible” realm of the forms, but the perilous land of “Faërie,” a source of song, of enchantment, of desire, of light.
The Flame Imperishable 