Knowing Fish

One day Chuang Tzu and a friend were walking by a river.
"Look at the fish swimming about," said Chuang Tzu, "They are really enjoying themselves."
"You are not a fish," replied the friend, "So you can't truly know that they are enjoying themselves."
"You are not me," said Chuang Tzu. "So how do you know that I do not know that the fish are enjoying themselves?"