"Ah, now, I'm glad you asked me that. It was one of my more brilliant ideas, and between you and me, that's saying something. You see, only one who wanted to find the Stone -- find it, but not use it -- would be able to get it, otherwise they'd just see themselves making gold or drinking Elixir of Life. My brain surprises even me sometimes.... "

Harry Potter and the Philosopher's Stone - Chapter 17 : The Man with Two Faces
See this quote in the Pensive -