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In 1277, Etienne Tempier was the bishop of Paris, and there was talk that, at the Sorbonne, members of the arts faculty - the professors of the non-theological side of the school - were teaching heretical ideas, mostly derived from Aristotle's writings. The Pope himself, a former Sorbonne theology professor, had written to ask Tempier to look into these rumors.
The bishop responded with a list of 219 propositions that he condemned as heretical. Any arts faculty who taught them would be excommunicated from the church and would lose their livelihoods as professors.
This 13th century disagreement between two university departments, arts and theology, would prompt medieval thinkers to consider ideas that might seem surprisingly modern. By rejecting a key Aristotelian principle, Tempier inspired later medieval scholars to develop a multiverse theory and to consider the possibilities of faraway planets and alien beings. From knocking out of scientific dogma, new ideas began to grow.
In the 19th century, Pierre Duhem, a scientist and historian, re-examined the history of medieval scientific thought and came up with a controversial thesis: there was, essentially, no "scientific revolution" during the Renaissance, only a continuation of work that was already happening in the "dark ages" of medieval thought. In particular, Duhem thought that Tempier's 1277 condemnations liberated Europe's Christian thinkers from Aristotle and opened up the way to the development of modern science.
Conventional historians sometimes take a skeptical view of Duhem, but in the case of medieval multiverses, at least, it's possible to follow a trail from one of Tempier's condemnations to ideas that emerged more than a century later about infinite worlds, full of alien creatures.
Among the ideas that Tempier condemned was a principle of Aristotelian thought that held that the "first cause" (or, as medieval scholars would have said, God) could not have made more than one world. The logic went something like this: Earth was among the world's four key elements, and one of its principles was that it moved towards the center of the world. If there was a neighboring world to ours, though, with earth at its center, that earth wouldn't be moving towards the center of our world. Since that violated the rules of how earth behaved, there could only be one world.
To Tempier, this idea went against a key theological principle: God was all-powerful and could accomplish whatever he willed. Since there couldn't be limits to God's power, there could be multiple worlds, if he wanted to make them.
Some medieval thinkers took this as a challenge. They started to look more closely, for instance, at previous Aramaic comments on Aristotle and considered what else might be possible. They found new ideas that were outside the bounds of the Aristotelian physics of the day.
Richard of Middleton, for instance, who lived in the second half of the 13th century, responded to Tempier by affirming that it could be possible to have more than one universe: "God could have and could still now create another universe." He tried to reconcile this with Aristotelian thought by arguing that the matter of a second world would stay in its own separate universe, and earth elements would gather at the center of each.
A later scholar, William of Ware, developed this idea further. What did it mean to talk about another world, he wondered? He didn't think it was possible to have two neighboring universes: by definition, the universe should include all the creatures ever made. So how could there be more than one? He argued instead that multiple worlds would have to be entirely separate, with no way of interacting - what today we might think of as parallel universes.
By the 15th century, medieval ideas about the universe had spun far from Aristotle's idea of a single world, with earth concentrating at the center. The theologian and astronomer Nicholas of Cusa, who lived from 1410 to 1464, believed that if you were able to leave the earth, you would find multiple luminous bodies existing alongside our own world - far-off stars, planets and moons. He ever went so far as to imagine that these planets might be inhabited: he thought the sun might have bright, intellectual inhabitants, whereas the moon might have a "lunatic" population. This was still about a century before Galileo would famously reject the idea of a geocentric world and put the sun in the middle of the universe.
These medieval thinkers were working from a religious idea about divide power. But this line of inquiry prompted a scientific openness, too, to different ideas about the physical world and how it might work. Following Tempier's prompt led medieval scholars to some surprisingly modern ideas about parallel universes and exoplanets.
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