Platonic Solids

Plato and his disciples were so fascinated by the idea that there were only 5 regular polyhedra that each associated to a natural element: the tetrahedron with the fire, the Hexahedron to the Earth, the octahedron to the air, the dodecahedron to the cosmos and the icosahedron to water. (source)

Plato believed that geometry hid sacred truths describing reality. They say that when he founded the School of Athens in 387 BC he placed a sign at the entrance that said: "Let no one ignorant of the geometry between here."