Fun story. After years of meditating on the topic, the only magic system that's 100% consistent is conjuration, and it has deep roots in religious mythos. Magic is, at its core, any alteration of natural laws. Natural laws, in turn, are the expression of divine (spiritual) power and authority. The divine spirit commands the world to operate in a particular fashion, and it is so. Ergo, in order to cast magic, you must communicate with the divine spirit and persuade it to heed your words. However, it is more feasible to communicate with its subordinate servants, namely angels and demons, who inherit divine authority over the world. This is where conjuration comes in - you summon one of these higher spirits and trade with them something of value in exchange for the use of their authority. Angels, being divine servants, cannot be summoned by any but their divine master, so the only spirits humans can feasibly summon are demons - and even then, being lesser creatures, we must trade at a disadvantage. That is why magic is perceived as dangerous IRL.

All the trappings of magic point toward conjuration. Runes are written language, but stones do not understand language. Minds understand language. Spell chants are spoken language, but rocks don't have ears and can't listen. Minds can listen. Elemental bending and dances are a type of sign-language, but the earth can't see. Minds can see. If you want to move something without touching it, something else must touch it to apply that force - and if you want that something to be nothing, that would be an alteration of natural law. Any kind of magic that involves communication, or even telekinesis, necessitates a sapient, miraculous response, implying spiritual operation. Thus, all such magic is conjuration.

So, if we want to step away from religious mythos to give our magic system a more natural bent, we'll have to get creative. A simple method is to grant humans the same kind of authority over the world that angels and demons have. We could treat the divine spirit like a natural power, kind of like the Force in Star Wars, to eliminate having to bargain for power. And if we want to steer clear of the danger involved in conjuring demons, we could make the spirits largely neutral, like sprites or fae.

Thus can we develop the three schools of magic:
* Wizardry (wizard), for invoking divine power directly;
* Conjuration (warlock), for invoking divine power through its servants;
* Thaumaturgy (sorceror), for invoking divine power granted to oneself.

If we're doing wizardry, you'll need to communicate with the divine in a language it can understand. There's no reason it can't understand you in any way you choose to communicate, but if we want to make it challenging, we'd want it to understand only a particular language that you have to learn, such as runes, Latin chants, or bending.

If we're doing conjuration, you'll need to communicate with a higher spirit in a language it can understand. Again, there's no reason it can't understand whatever language you use, so for all the same reasons listed above, we'll give it some specificity. Since we're now dealing with an individual, sapient spirit, possession is a possibility; whether that's a good or a bad thing depends on what we're aiming for. You could also have it as a separate familiar. Either way, it's giving you its power, whether by controlling you directly or by granting you its authority.

If we're doing thaumaturgy, then remember, in order to move something without touching it, something else has to touch it. With thaumaturgy, that's your own spirit. So you have to move your spirit. That will induce an out-of-body experience. Death is separation of spirit and body, so thaumaturgy would risk your death. To prevent that in our tale and add a comprehensible cost, we could say "mana" is the binding energy you expend for spellcasting.

If we want to add in some kind of cost to the other two schools, mana could be a component element of your life force, in which case expenditure would age you - or possibly just exhaust you somehow (though that's more governed by bodily energy and the ATP chemical than anything else). Or it could be a new kind of substance, like aether in Final Fantasy.

So yeah. It gets fun trying to rationalize everything.

Anyway, I'm going to try to keep this shortish because trying to type it up in real big detail was starting to disintegrate my brain. Essentially, all of existence runs on something called 'the code', which is basically like computer code, but for every inanimate object, living thing, and even metaphysical concepts of things like death. The three schools are the current known ways to utilise the code, but in reality it's only the tiny collection of information people have managed to put together in a coherent way, as the true nature of the code is so vast and incomprehensible that it's entirely possible it might just start to contradict itself one day, and no one would understand why. This is most clear in the fact that the code may actually be alive itself, but nobody knows how they would even begin to test that.

The first of the three schools is Thaumaturgy, which involves using runes to tweak and alter the code of whatever they're written on directly, but have to be done absolutely perfectly or else bad times for everyone.

The second is Shamanism, which involves seeking aid from spirits. Spirits are actually parts of the code that have 'awakened' though strange, often unknown circumstances and become real souls like those of other sapient entities.

Finally, the third is sorcery, which involves using your soul to perform various effects. To do this, a mage has to learn their True Name, which is basically the code of their soul in it's written form. It's a complicated, dangerous form of magic, and making a bad enough mistake can cause irreaprable damage to your soul. However, it's still easier to do safely than the more complicated Thaumaturgy, if not a little less versatile due to having to meditate on certain concepts and attune your soul to them to cast spells of their type effectively.

Anyway, those are the basics of it. It does actually does go a lot deeper than that, but I guess I never actually wrote it down before now, so it's all just a tangled up mess in my brain, and when I tried to sort it out and write it fully before, said brain decided to try and melt itself. So, instead of sticking to that one, I moved it to a google doc to sort out some other time I'm feeling a little too relaxed, and instead tried to summarise it all as simply as possible with this new version. It went... ok...