“If a person should ask my advice before undertaking a long voyage, my answer would depend on his possessing a taste for some branch of knowledge” .
At present there is a popular consensus in favor of intelligent life elsewhere in the universe. These notions are at least 2500 years old and rest on several assumptions, that have run as a common thread throughout history. Many lie outside the realm of scientific enquiry coming under the heading of “must be” arguments. The first is that the universe is infinite and so must contain planets identical to the Earth. This is often phrased as the “Big Numbers” argument. There are so many stars with planets that somewhere out there must be a replica of us. The second is that because life exists here, it must be common elsewhere. The third is that the development of intelligence is inevitable and happens elsewhere concurrently with, or more commonly in advance of, the evolution of life on Earth. These themes are addressed below under several headings.
THE PLURALITY OF WORLDS 
The discovery of many planets orbiting other stars, free-floating objects and the widespread occurrence of dusty circum-stellar disks, some with gaps in which planets are lurking, has raised once again in a dramatic fashion, the ancient question posed amongst others by Albertus Magnus in the thirteenth century: “since one of the most wondrous and noble questions in Nature is whether there is one world or many, a question that the human mind desires to understand, it seems desirable for us to enquire about it” . One of the favorite current quotations of astrobiologists comes from Metrodorus of Chios (350 BCE). It is usually stated as “it is unnatural in a large field to have only one ear of corn and in the infinite universe, only one living world”. These questions have been discussed under many headings for the past 25 centuries since Democritus, Epicurus and Metrodorus favored a multitude of worlds.