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Have boundless world is like, and this is different from our Earth. Atomic was for an unlimited number ... of its commitment to the distance, into space. For those who belong to this type of atomic nature of a world that can be created by them, or they made is not yet run out of the same world, or a limited number of the world ... so there are no longer an obstacle to an infinite number of the world ... We must believe that in all the world, all living creatures, plants and other things, we can see that in this world.           Of Atoms and Worlds     The idea that we may not be alone in the universe is not a new one. In the fourth century BC the Greek philosopher Epicurus wrote in a letter to Herodotus:   There are infinite worlds both like and unlike this world of ours. For the atoms being infinite in number... are borne on far out into space. For those atoms which are of such nature that a world could be created by them or made by them have not been used up on either one world or a limited number of worlds... so that there nowhere exists an obstacle to the infinite number of worlds... We must believe that in all worlds there are living creatures and plants and other things we see in this world.   Thus the notion of the plurality of inhabited worlds dates back to the very dawn of rational thought and scientific inquiry. This is all the more remarkable given the fact that Greek cosmology, and other early models of the universe, bear little resemblance to the modern scientific picture of the universe.   In the absence of proper empirical astronomical research, Greek speculations about extraterrestrial systems rested almost entirely on philosophical debate, so there was plenty of room for dissent. Aristotle, for example, rejected the concept of other worlds outright: "The world must be unique," he wrote. "There cannot be several worlds."   Justification for belief in other worlds was closely associated with the philosophy of atomism, initially expounded by Leucippus and Democritus, according to which the cosmos consists of nothing but indestructible particles moving in a void. As all things are made of atoms, and atoms of the same class are identical, it follows that similar associations of atoms to that which forms the Earth may also form elsewhere in the void:   The worlds come into being as follows: many bodies of all sorts and shapes move by abscission from the infinite into a great void; they come together there and produce a single whirl, in which they begin to separate, like to like.   This account of formation of other worlds is attributed to Leucippus by the third-century historian Diogenes Laertius.   Belief in the plurality of worlds was also adopted by the Roman poet and philosopher Lucretius. Also an atomist, Lucretius repeated Epicurus' argument, that given an infinity of atoms, there is no obvious hindrance to the formation of other worlds: "when abundant matter is ready, when space is to hand, and no thing hinders" then other worlds will naturally form.... Here in antiquity was the essence of an argument that lies at the heart of modern SETI [Search for Extraterrestrial Intelligence] research. Given an abundance of matter and the uniformity of nature, the same physical processes that led to the formation of the Earth and solar system should be repeated elsewhere. And, given the appropriate conditions elsewhere, life and consciousness should emerge on other worlds in roughly the same manner as they have emerged here.   It is in the highest degree unlikely that this earth and sky is the only one to have been created...This follows from the fact that our world has been made by the spontaneous and casual collision and the multifarious, accidental, random and purposeless congregation and coalescence of atoms whose suddenly formed combinations could serve [to produce]... earth and sky and the races of living creatures.   The Greek atomists were open-minded about whether other worlds had life on them. The idea of extraterrestrial life was in any case a common topic of discussion among the ancient Greek philosophers.         — Paul Davies, Are We Alone?: Philosophical Implications of the Discover of Extraterrestrial Life                       Indexes/17