Now on towards spoilerville. Like I said four years ago (seriously, that long?):
And here we are at last, early in Game 2–at least, on my laptop. I’m still a ways off having a playable draft to post on Dashingdon, I’m afraid. (Who knew that this would be the year for simultaneously restructuring a $10m-a-year charity, fighting a Supreme Court battle against a billionaire, and organizing the pushback against a flood of poorly thought-through government policies, all at the same time?) Until real life gets boring enough again for me to make more progress on the draft, I’ll just have to pass on occasional lore.
In this case, I’m drawing from the infodump on Theurgy you get from Cerlota de Viore when you meet her in the Xaos-Lands. (Which everyone does, whether or not you actually learned about her from Horion in Game 1.)
Theurgy isn’t based on thoughts or brainwaves. Jumping back again to cryptic things I said a couple of years ago:
To unpack that (and seriously, y’all, here be big fat grimdark spoilers): if you asked most people in the Hegemony, “what does the ‘aetherial’ in aetherial blood mean?” they’d say it’s basically a synonym for Angelic. It doesn’t have any particular meaning other than “holy and weird like the Angels.”
Only a few are aware that it’s not originally a religious term, but alchemical vocabulary that crept partway into popular usage (without laypeople ever quite understanding what they were talking about)…before it became suddenly clear to the relevant alchemical elite that they needed to make sure that no one outside a very limited circle of practitioners ever found out what “aetherial” really meant. It was too late to keep people from saying it, but not too late to mystify the term beyond recognition.
As that small cadre of early alchemists discovered when they began successfully probing the reasons Theurgy worked, human blood has a relatively low aether concentration, which it loses easily by sublimation once the blood leaves the body. A living person using their own living blood makes fairly effective use of its teleomorphic potential. Early alchemists tried and failed to preserve that potency through preventing sublimation. None were able to come up with anything that fixed a meaningful volume of aether, not enough for more than a party-trick level of Theurgy–nothing that could really compete with a dedicated Theurge using their own blood.
Thaumatarch Hera was thought to have been the one who finally cracked that problem, through a carefully guarded refinement process. In fact, while she did discover a new alchemical pathway for reducing sublimation, her truly crucial insights were (1) that other human bodily tissues have a much higher aether concentration than blood (If you go kamikaze and pull down a mountain in Game 1, you’re basically drawing on a whole bunch of that aether, not just the stuff in your bloodstream) and that (2) it would be possible to mask a fairly large-scale process of harvesting those tissues thanks to the Krypteia traditions of helot-hunting in certain city-states.
Thus began the conquest, and the Thaumatarchy.
Those aether-dense tissues (brain, mostly, but the eyes also have enough to be worth harvesting) lose less to sublimation before the aether can be “fixed” through the alchemical process that begins in the Harrower (yielding an inert precursor that can later be refined into usable form). Blood remains a useful natural solvent, contributing a bit to the solution’s potency… and “blood” makes an even more useful, politically tolerable label for the final product.
And yes, the aether concentration in humans does drop off significantly from infancy through puberty before stabilizing in adulthood. The Karagond Theurges pride themselves on their refusal (by and large) to make use of that fact; you’ve got to have something to point to that makes you feel less of a monster at the end of the day. Of course, they can’t overtly explain to the laity just why they’re so proud of eschewing child-murder, but they spare no opportunity to blast the Halassurqs for it. When you meet Erjan in Game 2 Ch 1, you can hear how Halassur justifies the opposite choice, while abhorring the nightmare slave system of the Thaumatarchy.
What does it mean that human beings are the only known sublunary source of the element that makes telos malleable (by analogy to the way the element fire makes many lesser substances malleable)? Well, some–notably Abhuman philosophers and seers, and some long-extinguished Karagond heretics–would suggest that this is strong evidence that humans were wrought by Theoi or Angels with a particular overarching end in mind, as a couple of folks surmised over on the old WiP thread. This is of course rankest heresy, unwhisperable among Karagond Theurges.
It’s also not the only explanation on offer. Nyrish Theurges continue to think any “argument from design” is bollocks, and kenonite or otherwise skeptical MCs will be free to agree.
So there you go. That’s what everyone will learn about halfway through the first chapter of Game 2. 
Please do spoiler-tag your reactions…but after all these years of hinting, I’d welcome your thoughts.