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Cosmic pluralism, the plurality of worlds, or simply pluralism, describes the belief in numerous other worlds beyond the Earth which harbour extraterrestrial life. The debate over pluralism began as early as the time of Thales and has continued, in a variety of forms, until the modern era.

In Greek times the debate was largely philosophical and did not conform to present notions of cosmology. Cosmic pluralism was a corollary to notions of infinity and the purported multitude of life-bearing worlds were more akin to parallel universes (either cotemporaneously in space or infinitely recurring in time) than to different solar systems. After Thales and his student Anaximander opened the door to an infinite universe, a strong pluralist stance was adopted by the atomists, notably Leucippus, Democritus, and Epicurus. While these were prominent thinkers, their opponents—Plato and Aristotle—had greater effect. They argued that the Earth is unique and that there can be no other systems of worlds.[1] This stance neatly dovetailed with later Christian ideas[2] and pluralism was effectively suppressed for a millennium.

Eventually the Ptolemaic-Aristotelian system was challenged and pluralism reasserted, first tentatively by scholastics and then more seriously by followers of Copernicus. The telescope appeared to prove that a multitude of life was reasonable and an expression of God's creative omnipotence; still powerful theological opponents, meanwhile, continued to insist that although the Earth may have been displaced from the center of the cosmos, it was still the unique focus of God's creation. Thinkers such as Johannes Kepler were willing to admit the possibility of pluralism without truly supporting it.

During the Scientific Revolution and the later Enlightenment, cosmic pluralism became a mainstream possibility. Bernard le Bovier de Fontenelle's Entretiens sur la pluralité des mondes (Conversations on the Plurality of Worlds) of 1686 was an important work from this period, speculating on pluralism and describing the new Copernican cosmology.[3] Pluralism was also championed by philosophers such as John Locke, astronomers such as William Herschel and even politicians, including John Adams and Benjamin Franklin. As greater scientific skepticism and rigour were applied to the question it ceased to be simply a matter of philosophy and theology and was properly bounded by astronomy and biology.

In the late nineteenth and twentieth centuries the term "cosmic pluralism" became largely archaic as knowledge diversified and the speculation on extraterrestrial life focussed on particular bodies and observations. The historic debate continues to have a modern parallels, however. Carl Sagan and Frank Drake, for instance, could well be considered "pluralists" while proponents of the Rare Earth hypothesis are modern skeptics.

• Extraterrestrial life
• Extraterrestrial life in culture
• Mediocrity principle
• Planetary habitability