In Puella Magi Madoka Magica, before Ultimate Madoka's universe, I read that the system where a Magical Girl would become a Witch was referred to as the "Law of Wheels", while after that, the new system and Madoka herself is known as the "Law of Cycles", this term actually being used by Homura and the Incubators in the Rebellion movie.
I am wondering, where did these terms come from? Neither system seems like a wheel/cycle as in neither one does a Magical Girl ever return to normal (the origin).
EDIT: I seem to have made a mistake as Mami may have said "Law of Wheels" in Episode 12 in Ultimate Madoka's Universe, but still it doesn't change my question of where these terms came from.