More specifically in regards to the book... there's three energetic forms within the person: The mental circuit, the physical circuit and the spiritual one. They each have separate roles and metaphysical meanings. Psychic power stems from the mental circuit, magical power from the physical one.
The interconnection and interplay between the three forms a fourth circuit and that set of interconnections is the soul.
There's a bit more to it than that, but that's the gist of it.
So, the soul is more or less as much of the body as anything?
What happens if you were to, say, transfer a mind to something else?
A mind can reach out. A psychic can move his mind out, but he'll always keep a tether to his original body; relaxing his willpower would cause his mind to instantly snap back into his prior body. The mind could be trapped and the body would function solely on its reptilian and instinctive part, heartbeat, breathing, digestion etc. Once the entrapment was broken the mind would again snap back, provided the body wasn't dead.
To break that tether one would have to kill the original body. On death the two other circuits... move on, to use the common parlance. The mental circuit will eventually find its way back, generally. You can't actually break the links between the three separate circuits, because that's tantamount to literally tearing the soul apart. I mean... Yeah, I'm just going to straight up say you can't do it, because if I say you can then BFEL will try it for munchkining himself into a lich or something. No, BFEL, you cannot become a lich. Or a ghost, servitor, immortal, gargoyle, archdemon, golem or anything else that will convey immortality and/or unfathomable power on you.
Which one is the "White" Circuit?
Also, magic comes from the physical one? Interesting, I shall have to meditate on the meaning of this. Actually since magic comes from the physical and psychic power is from mental does that mean a mage can't read minds?
What connection do Light/Shadow have to these "circuits"?
Are all the novels gonna explore a different one of these pieces of the soul? Cool premise.
The White Circuit is, to put it simply, a fourth piece of the soul. It's an addendum a select few people have in their souls which grants them the ability to heal anybody who doesn't possess the same white circuit. There will be explorations of each part of the soul, but not in-depth; the books focus mostly on the soul as a whole.
Yep, magic is physical. Why do you think it hurts so much? No, mages can't read minds.
In-depth knowledge of the circuits is not common. You would know (being a healer, but spiritual healings are... quite advanced), Lyu probably knows the outline, Ekoka would have a fair understanding, but most people just sorta accept that your soul moves on to whereverafter when you die and that poking it is a bad idea.
Honestly, I don't know the connection between Light and Shadow and the soul. I imagine if someone was spiritually wounded, you could blend a little light and shadow with your life magic to heal them. Or maybe a little force? I imagine there's a similar potentially representative blend to heal minds, too. Who knows. Again, I dreamt up the eight-element split for the sole purpose of this game. The split didn't occur before the game started, although the same constraints and laws still existed; no creation or destruction of matter, it's painful and will one day kill you (Seriously, it's the leading cause of death for mages) and so on. About the only bonus for mages is they tend to have supercharged immune systems.
Their white blood cells know fireball.
Strictly speaking, in the book world mages are instruments of order. Those more predisposed to imposing order tend to have magical power. Lots of mages serve in police forces for that reason. Every now and then, there's a prophecy of somebody who's going to come in and toss everything into the air, as all good prophecies do (There's never one of "This man shall appear and be
really good at taxes). That's never a mage. But there will almost always be a mage who will help them, who will temper their natural chaos with his natural order. Hopefully guide it into something productive rather than 'I stab everybody now'.