1Kyros: Anyway, ignoring Draak's obviously ridiculous statement, the four elements of fire, water, earth, and air make up all material objects.
2 Kyros: It is the elusive fifth element, of which I seek knowledge, that imbues immaterial properties.
3 Kyros: Such as time, space, the heavens, gods, and the souls of the living and the dead.
4 Lambert: You want to get into the unspeakable evils of necromancy?
4 Kyros: And other sorts of magic!

In the classical system of the four elements which Kyros names, a later development by Aristotle was the addition of a fifth element, which was usually called aether. Aristotle felt a need for this to distinguish the motions of stars and planets seen in the heavens from the motions of earthly objects.

His argument was that the four original elements—earth, fire, water, air—were changeable, and moved in straight lines (unless prodded by outside influences). But these qualities did not seem to apply to the heavens, in which the sun, moon, stars, and planets wheeled around the Earth in perfect cycles, never ending or changing. So, figured Aristotle, they can't be made of the same sort of stuff we have on Earth. Therefore they must be made of a fifth material, which has the properties that it never changes, and that it moves constantly in circles.

All very logical, from a 4th century BC Greek perspective.

By the European medieval period, Aristotle's teachings were looked upon with the reverence of ancient wisdom and authority, and they formed the underlying basis of church doctrine about the physical world. So in this climate it was difficult to assert anything else.

Although modern science has revealed the physical nature of our universe to be very different from what Aristotle espoused, his original elemental system makes a good basis for fictional fantasy magic systems, and is still very popular for that purpose.