plurality of worlds (pluralism)

In an astronomical context, pluralism, or 'the plurality of worlds,' is the belief that there exist numerous other worlds harboring life and, in particular, intelligent life. The debate over pluralism extends back at least two and a half thousand years, to the time of ancient Greece, and continues vigorously today. For most of this period, a lack of hard scientific data forced the discussion entirely onto a philosophical and theological footing. Only in relatively recent times have astronomical and biological considerations come to the fore.


The first strong pluralist stance was taken by the early proponents of atomism and its associated doctrines, notably Leucippus, Democritus, and Epicurus. The opposing view – that Earth is unique and that there can be no other systems of worlds – was championed by a number of other prominent Greek philosophers, including Plato and Aristotle. The possibility of lunar inhabitants was raised by Anaxagoras, Plutarch, and Lucian. It is important to recognize that "other systems of worlds" in the classical sense did not imply planets going around remote stars. As far as most Greeks were concerned, the stars of the night sky were small lights located in a vault that immediately surrounded the "sphere" of Saturn. The multitude of life-bearing worlds envisaged by the atomists was invisible and inaccessible from Earth, rather like the parallel universes of modern scientific speculation. These supposed other worlds might exist contemporaneously with the Earth (as the Stoics believed) or form a linear succession in time (as, for example, the Roman orator Cicero maintained). See ancient philosophy, related to the possibility of extraterrestrial life.


Throughout the early Christian era and into the late Middle Ages, there was little discussion of the possibility of extraterrestrial life, beyond that of angels and demons. The conventional geocentric model of the cosmos, based on the teachings of Aristotle and Ptolemy, suited the Church well. Allowing, as it did, for only one Earth and therefore a privileged position for mankind, it avoided serious theological concerns to do with the incarnation and redemption which surrounded the notion that there might be intelligent corporeal life-forms elsewhere (see Christian doctrines and pluralism). Despite these concerns, however, a number of prominent scholastics eventually came to question the Ptolemaic-Aristotelian cosmos in its original form. William of Ockham, Jean Buridan, Nicole Oresme, and William Vorilong were among those who pointed out that Aristotle's insistence on the impossibility of there being more than one kosmos placed unacceptable restrictions on God's power. An omnipotent Creator, they said, must at least be credited with having had the option to make multiple worlds - a point of view officially sanctioned in 1277 by Etienne Tempier, the Bishop of Paris, acting on papal authority. With this change to the Aristotelian scheme in place, the consensus became that while God could have created other inhabited worlds, in reality He chose not to do so. But then, in 1440, Nicholas of Cusa took a bolder step by arguing that whatever lay in God's power must have been realized – a principle that has come to be known as plenitude.


Following the demise of the geocentric theory at the hands of Copernicus, and the realization that the Earth is just another planet, speculation about life on other worlds, both within the Solar System and beyond, flourished. Bruno, Kepler, Campanella, Wilkins, Borel, Cyrano de Bergerac, Huygens, Fontanelle, and others, used the heliocentric world view as a logical basis for extraterrestrial speculation. For the first time, astronomical observation, made with the help of early telescopes, began to play a part in the pluralism debate. However, theological and philosophical arguments continued to hold sway. Those opposed to pluralism insisted that although the Earth may have been displaced from the center of the cosmos, humanity and its planet were still the unique focus of God's attention and love. Conversely, post-Copernican pluralists saw multiple life-bearing worlds as an expression of God's creative power and generosity. Other pro-pluralism arguments included analogy, belief in a cosmic chain of being, and teleology. By the second half of the 19th century, science began to play a far more decisive role in the pluralism debate, leading to a new climate of skeptical and rigorous inquiry.


Significant advocates of pluralism
Pre-17th century 17th century 18th century 19th century
Anaxagoras Addison, Joseph Adams, John Arago, François
Bruno, Giordano Bentley, Richard Alembert, J. le Rond d' Ball, Robert S.
Campanella, Tommaso Bergerac, S. Cyrano de Baker, Henry Balzac, Honoré de
Democritus Borel, Pierre Barthélemy, Jean J. Bernadin S-Pierre, J.
Epicurus Fontanelle, Bernard Beattie, James Brewster, David
Kepler, Johannes Godwin, Francis Berkeley, George Chalmers, Thomas
Leucippus Huygens, Christiaan Bode, Johann Elert Davy, Humphry
Lucretius Locke, John Bolingbroke, Henry De Concilio, Januarius
Nicholas of Cusa Wilkins, John Bonnet, Charles Dick, Thomas
Teng Mu   Boscovich, Roger J. Flammarion, Camille
Xenophanes   Buffon, Georges, L. L. Gauss, Karl Friedrich
    Derham, William Gruithuisen, Franz
    Herschel, William Helmholtz, Hermann
    Kant, Immanuel Herschel, John
    Lalande, Jérôme Littrow, Joseph J. von
    Lambert, Johann H. Olbers, William
    Lomonosov, Mikhail V. Procter, Richard A.
    Schröter, Johann H. Reynaud, Jean
    Swedenborg, Emanuel Secchi, Pietro Angelo