Douglas R. Hofstadter and Daniel C. Dennett
Publisher: Bantam Books (1982)

Two sages were standing on a bridge over a stream. One said to the other, “I wish I were a fish. They are so happy!” The second replied, “How do you know whether fish are happy or not?” You’re not a fish.” The first said, “But you’re not me, so how do you know whether I know how fish feel?”