To know fish.

One day a Zen master and his friend were walking near a river.

The Zen master gazed into the river and said, "Look at the fish swimming around; they are really enjoying themselves."

"But, you are not a fish," replied the friend, "So you can't truly know that they are enjoying themselves."

"You are not me," said replied the Zen master. "So how do you know that I do not know that the fish are enjoying themselves?"