I don't really want to use Mage, because I have played it a bunch, and it gets into a whole bunch of questions about the nature of reality and people have a really, really hard time grasping how paradigm works. Paradigm becomes a big pain, conceptually. I also don't mind if someone does what would be vulgar magic in this game.
To me, it is simple enough: magic exists. It is a thing. We cheat in that the world isn't really changed by it (no magic technology, for example) but accept that it exists in the capacity that people have always believed it to. People who use magic are around and doing things, and people believe in it, but there also isn't a mage school or anything - there are the secret societies and cultures we discussed. There are many different forms and ways to make it work, but people with the talent and focus can use various forms of magic. Now, can people throw fireballs and stuff like that? Not really. Magic works more in the way people have believed it to work, as rituals and talismans and methods that let you do a wide range of effects, but usually at a cost or for a limited time. Magic would be bounded only by the system in which it works; if you were trained as a hex-throwing Celtic witch, you wouldn't be doing Hermetic summoning. It is like different forms of cooking - a French-trained chef typically uses those methods to make meals, even if they dabble in others.
In terms of power, that is completely handled by us narratively and how I set difficulties. Divining the location of an item might not be that hard; if you want to summon a demon, that is harder; if you want to sink a continent into the ocean, you are looking at nearly impossible without the right tools and a lengthy quest.
