The contemplation of the heavenly vault suggests to any generalizing intelligence the idea of the world suspended within this concavity. During rude barbaric times such as the High Middle Ages – the records of which are precious in this respect – it was thought that this vault rested on the surface of the Earth like a glass bell. And when experience gained firstly by terrestrial travels and afterwards by circumnavigation showed that this was a delusory phenomenon and, at the same time, that the Earth was an autonomous sphere floating within space, the [heavenly] vault became in turn a hollow and translucent sphere containing the concentric world, like the white of an egg surrounding the yolk. Cosmological experience then revealed that the stars were situated at different heights in the supposed vault, which compelled imagining new concentric spheres. Finally, it was discovered that there are neither such spheres nor such a vault; that the amplification and multiplicity of the latter were illusions – as was the concave aspect of the sky – and that the space enclosing the universe is an abyss.