My setting search entry used the idea that all magical energy came from the gods
originally. There's sort of a migration path of magical energy
divine <--> druidic <--> arcane.
The stripped down, short version is that the first war of the gods wrought such destruction on the world, that it was in danger of unraveling. The First god and goddess (who had created the world) ended up immolating themselves and binding a great deal of their power to the Earth (it's actually a LOT more complicated than that, but I'm going against type and trying to be semi-brief

). Besides ending the war, this gave the planet the energy needed to sustain itself, and something of a will of its own, derived from the creators. It could choose mortals (druids) to act as instruments of its will and wield its powers. The power of the earth is tightly bound and seemed self-regenerating, but subsequent wars of the gods, powerful mortals, dragons, etc. (There's a long history) destabilized the ley, and it leaked out to saturate the world. Arcane casters tap this loose ley energy.
And there's sort of a push-pull relationship. The magic that leaks out into the world diminishes the power of the Earth. When the magic of the Earth grows too weak, it must be re-inforced, or the world will die. This has had to be done a few times over the course of history, but requires a significant input (read: sacrifice) of divine or arcane energy. Most recently, a goddess descended to earth and sacrificed her divninity and nearly all of her power to "recharge" the ley and all but seal the "leaks" (she created the fey in the process, and became their queen, leading them in their chores of caring for nature. They're fickle, but at least they proved better stewards for the world than the dragons, but that's a whole 'nuther story). Of course, "sealing up the cracks" in the ley diminished the flow of arcane magic, which sure didn't please arcane casters. Or the Empire built dependent on the vast amount of free arcane energies.
There's also a Source out there from which the Creators themselves sprung, which is limitless, but is beyond mortal ken and cannot be tapped. Even the remaining gods cannot use it, and only the wisest of them has any understanding of it.
That's probably not enough explanation to make things clear, but it's getting late.