Belief in extraterrestrial life may have been present in ancient Egypt, Babylon, and Sumer, although in these societies, cosmology was fundamentally supernatural and the notion of aliens is difficult to distinguish from that of gods, demons, and such. The first important Western thinkers to argue systematically for a universe full of other planets and, vicariously, possible extraterrestrial life were the ancient Greek writers Thales and his student Anaximander in the 7th and 6th centuries B.C.E. The atomists of Greece took up the idea, arguing that an infinite universe ought to have an infinity of populated worlds. Ancient Greek cosmology worked against the idea of extraterrestrial life in one critical respect, however: the geocentric universe, championed by Aristotle and codified by Ptolemy, privileged the Earth and Earth-life (Aristotle denied there could be a plurality of worlds) and seemingly rendered extraterrestrial life impossible.

When Christianity spread through the West the Ptolemaic system became dogma and although the Church never issued any formal pronouncement on the question of alien life [2], at least tacitly the idea was heretical. In 1277 the Bishop of Paris, Etienne Tempier did overturn Aristotle on one point: God could have created more than one world (given His omnipotence) yet we know by revelation he only made one. To take a further step and argue that aliens actually existed remained dangerous. The best known early-modern proponent of extra-solar planets and widespread life off Earth was Giordano Bruno, who was burned at the stake for this and other unorthodox ideas in 1600.

The Church, however, could not contain the storm that accompanied the invention of the telescope and the Copernican assault on geocentric cosmology. Once it became clear that the Earth was merely one planet amongst countless bodies in the universe the extraterrestrial idea moved towards the scientific mainstream. In the early 17th century the Czech astronomer Anton Maria Schyrleus of Rheita mused that "if Jupiter hasâ?¦inhabitantsâ?¦they must be larger and more beautiful than the inhabitants of the Earth, in proportion to the [size] of the two spheres;" he did not dare to confirm the existence of Jovian beings due to potential theological difficulties. Later, this bold step would be taken. William Herschel, the discoverer of Uranus, was one of many 18th-19th century astronomers convinced that our Solar System, and perhaps others, would be well populated by alien life. Other luminaries of the period who championed "cosmic pluralism" included Immanuel Kant and Benjamin Franklin. At the height of the Enlightenment even the Sun and Moon were considered candidates for hosting aliens. The Christian attitude towards extraterrestrials turned from denial to ambivalence. Theological criticisms had been partially stalemated by a critical counter-argument that had remained in the background since the pronouncements of 1277: God's omnipotence not only allowed for other worlds and other life, on some level it necessitated them.