The horns of elfland

"Yes, I know what you mean," said Tom. "That's why I threw a stick in Dozmary Pool. I knew it was a silly thing to do. But I didn't want to just imagine the Lady of the Lake; I wanted to enter the story myself. There's a kind of yearning that comes over you that is also an ache. It's as if you've found the gate to Eden, but you're not allowed to go in."
    "That is it precisely!" boomed Lewis enthusiastically. I've been having that experience ever since I was a child. I call it 'Sweet Desire', or sometimes just 'Joy'. It's an ache for the infinite, a sense of some lost paradise calling out to us. Sometimes I hear the horns of 'elfland' in the call of birds flying overhead. Sometimes it can just be a phrase in a book -- 'the well at the world's end'."
  "Don't those experiences make more sense if you think of them as a kind of homesickness for Heaven? As Augustine said, 'Our hearts are restless until they rest in Thee.' If the Christian view is right, we are all exiles from paradise."

Excerpt from David C. Downing's Looking for the King