One day Chuang-Tzu and a friend were walking along a riverbank.

"How delightfully the fishes are enjoying themselves in the water!" Chuang-Tzu exclaimed.

"You are not a fish," his friend said. "How do you know whether or not the fishes are enjoying themselves?"

"You are not me," Chuang-Tzu said. "How do you know that I do not know that the fishes are enjoying themselves?"