In medieval philosophy, the earth was made of four elements—earth, water, fire, and air—but the vast universe beyond it was composed of a different substance entirely. This was aether, a particularly refined material that transcended the states we recognise. Known as the quintessence, it was neither hot nor cold, neither wet nor dry, and was capable of changing its density. It was the stuff from which stars were made, and light, and gravity. It tended naturally towards circular motion and so set the orbits of the planets.
