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13 Nicolas Copernicus, however unintentionally, gave plurality new life in 1543. His treatise “On the Revolutions of the Heavenly Spheres” proposed a universe in which the Earth and the other known planets revolved around the Sun, sending our world into motion and displacing the Earth and its inhabitants from the center of the cosmos. Copernicus did not invent this heliocentric conception of the universe; he knew that Aristarchus of Samos had proposed it long before. He refined and extended the idea through logic, mathematical calculation, and intuition—and very few observations." Copernicus has became a symbol, the man who ended one age and ushered in another. To many, he is the figure who led the West out of medieval obscurantism, giving us a more realistic conception of the uni- verse. In our eagerness to praise him, we should recall that he did not dis- mantle the entire Ptolemaic scheme. He held that the Sun was the center of the universe, not just of the solar system. The Copernican cosmos remained closed, bounded by a sphere of fixed stars.'’ Long-established conceptions of the universe that frame our sense of order are hard to kill. Nonetheless, Copernican theory had powerful implications for the debate about the multiplicity of inhabited worlds. It suggested that the other planets of our solar system were worlds rather than points of light on celestial spheres. Copernicanism also implied that there is no qualitative difference between one part of the universe and another. This did not prove that other planets were inhabited, but it did suggest that they might be inhabitable. Many people today think that Copernicanism demoted humans by removing them from their central pedestal. In fact, the Ptolemaic universe had placed the Earth at the lowest point in the cosmos, the least perfect level of existence; Arthur Lovejoy described it as “the dim and squalid cellar of the universe.”'* To remove humans from the center of things actu- ally raised them from their low estate. Religious authorities found heliocentrism less troubling than the plural- ity of inhabited worlds. It was not the position of our planet in space that gave the Earth its unique status and its unique share in the attention of Heaven, but the fact that it alone was supposed to have an indigenous population of rational beings. By implying that there might be many Humankinds, plurality subverted our uniqueness.'® Some sought to reconcile plurality with Christian doctrine by invoking what later became known as the Principle of Plenitude. The Creator’s omnipotence and freedom of action meant that whatever God can create will be created, including all possible worlds. Multiple worlds with sentient life-forms could be seen as demonstrating the wisdom and benevolence of God.” This theory did not resolve the underlying dilemma for Christian Copernicanism Copernicanism