The term historically used for the concept of other worlds, possibly inhabited, beyond the earth. The concept had its origin with the ancient Greek atomists Leucippus (fl. 5th century B.C.E.), Democritus (fl. late 5th century. B.C.E.), and Epicurus (341–270 B.C.E.), who held that an infinite number of kosmoi existed, while Aristotle (384–322 B.C.E.) argued for a single kosmos, with kosmos defined as everything in the visible world, including all celestial bodies. For both Aristotle and the atomists, their diametrically opposed conclusions were based on the physical principles of their cosmologies.
An entire medieval tradition of plures mundi was generated based largely on commentaries on the relevant portions of Aristotle’s De caelo (On the Heavens). By the end of the fourteenth century, these commentaries had transformed Aristotle’s conclusion of a single world to a position more in accord with Christianity: God could create many worlds if he wished, without suspending the laws of nature. But, in fact, the Scholastics inevitably concluded, God had not created more than one world.
The transformation in the sixteenth and seventeenth centuries to a plurality-of-worlds tradition, in which world meant an inhabited earthlike planet, was not achieved by successive rebuttals of Aristotle, even though the later writers were aware of the older tradition. Rather, the idea of inhabited planets stemmed from the Copernican revolution, in which the planets became earths and Earth a planet. Just as the heliocentric theory implied a new physics of motion—worked out by Galileo Galilei (1564–1642), Johannes Kepler (1571–1630), Isaac Newton (1642–1727), and their successors and examined in detail by historians of science as one of the hallmark achievements of the Scientific Revolution—so it also implied a new conception of the physical nature of the planets.
This new conception—extended also to the possible planets of other stars—was the essence of the renewed plurality-of-worlds tradition. But, while the motions of celestial bodies could be relatively easily observed, the physical nature of the planets and the existence of other planetary systems were not so amenable to observation, even after the invention of the telescope. It is this aspect that gives the subject added interest for the history of science, since the dearth of observational data led to an interesting interplay of philosophy, religion, metaphysics, and empiricism, driven by the Copernican theory. The idea of a plurality of inhabited earthlike planets began even before the telescope. Giordano Bruno (1548–1600), a dedicated but metaphysical Copernican, professed belief in an infinite number of worlds, based more on his philosophy than his Copernicanism. Kepler, too, in his Astronomiae pars optica (The Optical Part of Astronomy, 1604) and Somnium (The Dream, begun in the 1590s), speculated on the Moon as an inhabited world. But the telescope gave impetus to the idea that the planets were worlds similar to the earth, and Galileo’s observations, as well as Kepler’s speculations in his Dissertatio cum nuncio sidereo (Dissertation on [Galileo’s] Sidereal Messenger, 1610), began a line of research leading to the present day. This Copernican implication, however, required great caution in the Christian milieu. Galileo himself, while arguing that the Moon was a world, held that any lunar life would be far beyond our imaginings.
In contrast to Bruno, and despite Galilean caution, the Copernican theory was a crucial underlying concept that Kepler and others explicitly related to their belief in a plurality of worlds. John Wilkins’s (1614–1672) Discovery of a World in the Moone (1638) set forth all of the arguments for an inhabited Moon and answered scriptural objections. Bernard le Bovier de Fontenelle’s (1657–1757) extremely popular Entretriens sur la pluralité des mondes (Conversations on the Plurality of Worlds, 1686) used the Cartesian vortex cosmology to argue for the plurality of solar systems. And Christiaan Huygens’s (1629– 1695) Cosmotheoros (1698) illustrated the appeal of these ideas to one of the century’s most important scientific figures. The success of the idea of other worlds, however, was assured only when Newton’s successors made it an integral part of Newtonian natural theology. For most, the usefulness of inhabited worlds in manifesting the magnificence and omnipotence of God overcame all scriptural objections—at least for a while. The plurality of worlds was a compelling problem given serious and persistent attention in the seventeenth century. Although empirically beyond the limits of seventeenth-century science, it should be seen as an integral part of the Scientific Revolution, a completion of the process that began with the decentralization of the earth and the subsequent shift from the closed world to the infinite universe. The projection of mind into space, even if still unproved, was a watershed in the history of thought, which transformed the divine celestial regions of the medieval worldview into a universe filled with the rational intellect formerly reserved for man. The implications have since echoed through philosophy and theology, and, in the second half of the twentieth century, the idea of inhabited worlds led to major endeavors in astronomy and biology. Even though still unproved, it is a kind of “biophysical cosmology” that is increasingly testable, and whose final proof is even today viewed by many scientists as a completion of the Copernican revolution.