–Dumbledore, on the Mirror of Erised:

“‘It shows us nothing more or less than the deepest, most desperate desire of our hearts…However, this mirror will give us neither knowledge or truth.  Men have wasted away before it, entranced by what they have seen, or been driven mad, not knowing if what it shows is real or even possible.

‘The Mirror will be moved to a new home tomorrow, Harry, and I ask you not to go looking for it again.  If you ever do run across it, you will now be prepared.  It does not do to dwell on dreams and forget to live, remember that.'”

–from Harry Potter and the Sorcerer’s Stone by J. K. Rowling