I'd actually like to add my thoughts on the subject of the Arcanist's Aetheir source. By the real world examples of archain teachings like Alchemy, Sorcery, ect, ect (Weather you believe in them or not.) the basic rules for drawing energy for those spells or abilitys is know as "Energy Channeling" in the hermatic beliefs that most midevil real world magic was based off of. The short version would be that Humans (Or the humonoid races in Eorzia's case) would have very small natural reserves of Aetheir (The basic composition of the universe(s)) and those capable of using magic would be able to channel the aetheir around them into them selves to actually USE the magic.