One day, while him and his friend walking along the river surrounding the city, Zhuanzi(an ancient Chinese philosopher)pointed into water and said to his friend:" look, how happy the fish are swimming in the river." His friend replied:" you are not the fish, how could you know fish are happy in the water"? with a smile, Zhuanzi reciprocated:" My friend, you are not me, how could you know that I did not know fish were happy in the water".