Does an Invisible Light Exist?: The Empyrean Heaven and the Sphere of Fire: The Enigma of Luminous Bodies without Light

According to medieval cosmology, the universe contains two luminous bodies whose light is imperceptible: the sphere of fire and the empyrean heaven. Focusing mainly on some Sentences commentaries (from the 13th-14th centuries), this paper analyses different explanations given to the phenomenon of invisible luminosity. The above-mentioned cases call on the commentators to reflect on the relationship between luminosity and diaphaneity. Some of them introduce the notion of 'intrinsic luminosity' (natura lucis), a quality compatible with transparency: the empyrean heaven and the sphere of fire are therefore diaphanous and luminous, without shining. Others claim that a single body cannot be luminous and diaphanous at the same time. They therefore insist on the luminosity of the empyrean heaven, which remains less perceptibile due to the intense light of other celestial bodies. They also cast doubt on the very existence of the sphere of fire, arguing that if the upper fire existed, it would necessarily be visible, as it could be neuther diaphanous nor eclipsed by any obstacle.