There are infinite worlds both like and unlike this world of ours … We must believe that in all worlds there are living creatures and plants and other things we see in this world.
Epicurus (c. 300 bce)There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.
Shakespeare, Hamlet, Act I, Scene 5We shall not cease from exploration
And the end of all our exploring
Will be to arrive where we started
And know the place for the first time.
T. S. Eliot, Four QuartetsOne of the most basic questions that has been pondered by Natural Philosophers for (at least) the past few millennia concerns humanity's place in the universe: are we alone? This question has been approached from a wide variety of viewpoints, and similar reasoning has led to widely diverse answers. Aristotle believed that earth, the densest of the four elements, fell towards the center of the universe, so no other worlds could possibly exist. In contrast, Epicurus and other early atomists surmised that the ubiquity of physical laws implied that innumerable Earth-like planets must exist in the heavens.
Many aspects of the question of human uniqueness remain ill-constrained, but others have yielded to scientific investigation. Copernicus, Kepler, Galileo, and Newton convincingly demonstrated that the Earth is not the center of the universe, and that other worlds qualitatively similar to Earth orbit the Sun. Telescopic observations, and more recently interplanetary spacecraft, have told us a great deal about these neighboring worlds.