I think this is a question of definitions. Does the source of power have to be identifiable? Or can it be unknowable and ineffable?
I mean, from a DMing/world building perspective, yes it should be identifiable...even if that definition is "It's ineffable."
My world uses both.
Clerics (and other divinely powered classes) are - must, needs be- connected to an identified deific power (or collection thereof). A "God of such and such" for whom they are the spreader of their Word and exemplar of their Will, advocate, defender, champion (that deity's tenets and dogma) upon the world. That's part and parcel of what a Cleric is in the game world.
Druids -and those workers/users of "Nature Magic"- are connected to the mystical energies floating about and through and emanating from the "natural world," also called "the Green" or "the Balance," a.k.a. "Nature," writ large. They do not supplicate or invoke the God of Air, for example, to control the wind or entreat/contract with a Spirit of Beasts to speak to animals. They have the connection and training and knowledge to connect, harness, and direct the magical energies of/existing in Nature -call it a style of mysticism, I guess, if you want, as opposed to a "religion."
I'm presuming this is the kind of thing you mean as "unknowable/ineffable," a non-personified power that exists, that can be connected to, and utilized to make magic happen. Not a "god or goddess OF Nature" (which do also exist, are worshiped by mortals, and have/do empower priests...clerics of the Gods of Men -the world's existing pantheon. But Druids are NOT worshipers or champions of the "Gods of Men"), but Nature, itself.
But in a "meta" sense, at least imho, the DM should have an idea -however sketchy/hazy- of what/how these things work. So some degree of "identification/definition/explanation" should be available...both for players, and in-world consistency.