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A Very Short Introduction to the History of Catholic Debates About the Multiverse and Extraterrestrial Intelligence

Are we, on Earth, the lone intelligent inhabitants of this vast universe? The Catholic tradition teaches us that there are other rational creatures, namely angels, who are purely intellectual, non-physical beings. But do we humans share the cosmos with any other embodied intelligent forms of life?

Today, speculation about the existence of extraterrestrial intelligence (ETI) is livelier than ever in our culture. Yet many who contribute to this intensifying interest in ETI, especially Catholics, fail to realize that the contemporary discussion is only the most recent portion of a debate in Western thought that stretches back at least twenty-six centuries. Fathers and Doctors of the Church, Catholic philosophers and theologians, popes and bishops, friars and priests, scientists and saints have all taken part in the conversation.

Ancient Greek Cosmology and the Church Fathers

The conceptual foundations for the Catholic discussion of ETI were laid in the centuries before Christ among Greek philosophers in the sixth, fifth, and fourth centuries BC. The question of intelligent life beyond earth was at that time part of a larger discussion about what came to be known as “the plurality of worlds.” This notion originally referred not so much to multiple heavenly bodies within our universe (such as stars, planets, and their moons) but rather to multiple entire universes, all coexisting independently of one another, each cosmos with its own earth and celestial bodies.

Thinkers in the Greek philosophical tradition known as “atomism” concluded that there is indeed a plurality of such worlds. Plato and Aristotle rejected that idea, but Plato thought the stars were living creatures who had each been given a soul. In this way, we might say that Plato conceived of ETI in the form of living stars who moved across the sky.

We might also note that Aristotle once speculated about inhabitants of the moon, though this idea contradicted his conception of the lunar region as part of the unchanging portion of the cosmos. In addition, followers of the sixth-century Greek philosopher and mathematician Pythagoras believed that the moon is another inhabited world, though not its own cosmos.

Early Christian thinkers were aware of the ancient discussion among pagan philosophers about the plurality of worlds. They continued the conversation, adding their own insights derived from the apostolic tradition of the Church. But they were concerned primarily with speculations about multiple entire universes, presumably inhabited. This possibility they largely rejected, following the Platonist and Aristotelian schools, which had concluded, for various philosophical reasons, that the existence of more than one universe would be somehow less “perfect” than a single cosmos.

The common Greek cosmology placed Earth at the center of the universe, with the sun, moon, and stars revolving around it. The planets (literally, “wanderers”) were simply stars that “wandered” from the path of the other heavenly bodies. This cosmological model thus had no conception of solid or gaseous spheres that could provide a home to living creatures. For this reason, the notion of ETI in the sense of inhabited planets or even moons within our universe received little if any attention from most Christian thinkers until the thirteenth and fourteenth centuries, when these long-held but pre-Christian notions came into question.

One possible exception to this position is found in a letter written by Pope St. Clement of Rome in the first century. He spoke of “the ocean, impassable to man, and the worlds beyond it,” which are regulated by God’s laws. Origen, a third-century Christian theologian with some plural-world speculations of his own, suggested two possible interpretations for Clement’s words. One was that the St. Clement might have been referring to other parts of the Earth that we cannot reach because the ocean prevents such a journey. In this sense, “worlds” would be parallel to the expression used by later, ocean-crossing explorers when they referred to Europe, Asia, and Africa as the “Old World” and the Americas as the “New World.” The second interpretation suggested by Origen was that St. Clement thought “the whole universe of existing things” contained “other worlds”; he “wished the globe of the sun or moon, and of the other bodies called planets, to each be termed ‘worlds.’” If this is the correct interpretation, then here we have what might well be the earliest surviving Christian reference to other worlds within our universe, and even to worlds with inhabitants.

We should also note that Church Fathers such as St. Jerome and St. Augustine, although not speculating about ETI (given the limits of ancient philosophy), did affirm the existence of non-human, non-angelic forms of intelligence. They both cited evidence that at least some of the creatures appearing in pagan mythology (such as satyrs or fauns) might actually exist and live on Earth, and such creatures did not seem to cause them theological difficulty. This observation is increasingly important today, when the public ETI conversation has begun broadening to include other possibilities for non-human intelligence (NHI).

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