A man came upon a tall tower and stepped inside to find it all dark. As he groped around, he came upon a circular staircase. Curious to know where it led to, he began to climb, and as he climbed, he sensed a growing uneasiness in his heart.

So he looked behind him and was horrified to see that each time he climbed a step, the previous one fell off and disappeared. Before him, the stairs wound upward and he had no idea where they led; behind him yawned an enormous black emptiness.

This is from The Heart of the Enlightened, a book of spiritual stories by Anthony de Mello.