Looking for extraterrestrial life: The search is for societies that are willing to join the club

People have always wondered whether there are other people. Chosen from innumerable quotes that could be adduced, from classical Greek times to the nineteenth century, here is the thought of Teng Mu, Chinese philosopher of the thirteenth century, “Empty space is like a kingdom, and heaven and earth no more than a single individual person in that kingdom. Upon one tree are many fruits, and in one kingdom many people. How unreasonable it would be to suppose that, besides the heavens and earth which we can see, there are no other heavens and no other earths?”