The idea that there might be other inhabited worlds dates back thousands of years and has roots in both Eastern and Western thought. Doubtless it first took the form of beliefs in gods, goddesses, and other spiritual beings which inhabited realms beyond Earth. On a more intellectual level, Buddhism taught pluralism, as did some of the schools of Greek philosophy. Of the latter, atomism, as developed and propagated notably by Leucippus, Democritus, Epicurus, and Lucretius, is the most significant because the concept of multiple worlds and life is implicit in its cosmological scenario. Prominent among other ancient philosophers who speculated about the possibility (or impossibility) of other worlds and life, or who made discoveries relevant to these subjects, were

 

   Thales of Miletus
   Anaximander of Miletus
   Anaximenes of Lampsacus
   Xenophanes
   Anaxagoras of Clazomenae
   Plato
   Aristotle
   Aristarchus of Samos
   Eratosthenes of Cyrene
   Hipparchus of Nicea
   Plutarch
   Ptolemy
   Lucian of Samosata