We can often cling to our preconceived notions of truth, so much so that we can be blinded to the truth.

A great Zen example of this is when a man loses his home in a fire and finds a charred body which he takes to be his son and so he grieves for his son. One day, a few months later, there is a knock at the door and they claim to be his son. His son was kidnapped by bandits in the fire and he managed to escape. His father will not answer the door, convinced he has the body of his son who he has grieved for and mourned, his real son turns away and he loses his son forever.