The Buddha told a story to his monks:
A young widower was devoted to his little son. But while he was away on business, the whole village was burned to the ground by bandits, who took away the little boy. When the father returned and found only ruins, he was brokenhearted. He thought that the charred remains of an infant was his own child, so he organized a cremation, collected the ashes, and carried them always in a special bag
One day his real son managed to escape from the bandits and found his way back to his old home. His father had rebuilt the house. When he arrived, late one night, and knocked on the door, his father called, "Who is there?"
"It is I, your son. Please let me in."
The father, still carrying the ashes and hopelessly sad, thought this must be some wretched boy making fun of him and he shouted, "Go away!"
The boy knocked and called again and again, but the father always made the same response, and at last the boy left, never to come back again.
When he had told this story, the Buddha added, "If you cling to an idea as the unalterable truth, then when the truth does come in person and knock at your door, you will not be able to open the door and accept it."