Aether

[Enkidoh]

The very nature of aether it seems goes beyond being just mere 'life energy' - from info it seems that it is essentially the very energy of creation itself - the main 1.0 storyline pretty much stated this flat out, specifically talking about crystals, but by extension aether, particularly in crystal form, has the literal 'power to create', as mentioned here in this 1.0 storyline quest scene:

Minfilia: "I assume you are familiar with crystals - the elementally charged clusters of concentrated aether which craftsmen employ in synthesis?"

and

Nananoby: "While not as rare, you would be hard-pressed to find anyone who would not attest to the fact that crystals are often deemed more precious than even mythril. It is their power to create that makes them so valuable. There is a place in Mor Dhona called Silvertear Falls where crystals lie strewn about the land. However the corrupted wastes are crawling with Garleans, not to mention preternaturally strong beasts, driven mad by the land's taint."

This also neatly explains why the Primals require not only aether and crystals to exist, but also worship, as well as how beings like Good King Moggle Mog the XIIth, even an ACN's Carbuncle can exist despite not being a Primal - with enough aether/crystals and a strong enough will a person could make even the most abstract or intangible desire corporeal. Pretty much, a 'wish made manifest'.

Coincidentally, this could also explain how Disciples of the Hand actually 'craft' - despite appearances and the tools they're equipped with, they're actually not making an item by hand, but instead employ a magical shortcut using a crystal to basically 'will' that item into existence from raw materials, hence the term 'synthesis' (this also why crafting requires the use of a crystal) - this was actually how it was explained in FFXI (where synthesis indeed showed the player seeming to throw the ingredients together into a formless spinning mass in the air while they 'crafted', rather than using tools).

Of course this raises questions as to why Disciples of the Hand still require tools to be equipped, but it's possible they use a tool as some kind of conduit for the required thoughts or desire to make the object. But I'm getting ahead of myself here.