Cosmic pluralism, the plurality of worlds, or simply pluralism, describes the belief in numerous "worlds" (planets, dwarf planets or natural satellites) in addition to Earth (possibly an infinite number), which may harbour extraterrestrial life.

The debate over pluralism began as early as the time of Anaximander (c. 610 – c. 546 BC) as a metaphysical argument, long predating the scientific Copernican conception that the Earth is one of numerous planets. It 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 contemporaneously in space or infinitely recurring in time) than to different solar systems. After Anaximander opened the door to an infinite universe, infinite higher dimensions, and an infinite amount of universes and other classes of verses, a strong pluralist stance was adopted by the atomists, notably Leucippus, Democritus, Epicurus—whose Epistle to Herodotus clearly lays out the Doctrine of Innumerable Worlds—and Lucretius who elaborates this Doctrine in his work De rerum natura. Anaxarchus told Alexander the Great that there were an infinite number of worlds that each harbored an infinite variety of unknown natural phenomena and extraterrestrial life, leading Alexander to weep, for he had not yet conquered even one. 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. This stance neatly dovetailed with later Christian ideas, and pluralism was effectively suppressed for approximately a millennium.^{[citation needed]}

Many medieval Muslim scholars endorsed the idea of cosmic pluralism. Imam Muhammad al-Baqir (676–733) wrote "Maybe you see that God created only this single world and that God did not create humans besides you. Well, I swear by God that God created thousands and thousands of worlds and thousands and thousands of humankind."

Fakhr al-Din al-Razi (1149–1209), in dealing with his conception of physics and the physical world in his Matalib, rejects the Aristotelian and Avicennian notion of the Earth's centrality within the universe. Instead, he argues that there are "a thousand thousand worlds (alfa alfi 'awalim) beyond this world such that each one of those worlds be bigger and more massive than this world as well as having the like of what this world has." To support his theological argument, he cites the Qur'anic verse, "All praise belongs to God, Lord of the Worlds," in Surah al-Fatiha emphasizing the term "Worlds."

Two Qur'anic verses support the idea of God being Lord of multiple worlds: 1:2 and 41:09. Qur'an 16:8 says "He has created other things of which ye have no knowledge." However, some scholars argued that the expression used in the verses simply means "The Lord of all people".

Cosmic pluralism was depicted in fictional Arabic literature. "The Adventures of Bulukiya", a tale from the One Thousand and One Nights (Arabian Nights), depicted a cosmos consisting of different worlds, some larger than Earth and each with their own inhabitants.

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. Medieval philosophers like Nicholas of Cusa and Nicole Oresme wrote the possibility of the plurality of the worlds.