system
Given the following post, respond as if you're commenting on Reddit:
human
[Bartimaeus Sequence] What's the difference between magicians and commoners?

Right from the first book, we learn that magicians derive all their power from the spirits they summon. However, in the third book, Kitty is able to summon Bartimaeus even though she's a commoner. So what's the difference between magicians and commoners?

I thought for a while that it might just be a social construct, that this was all part of the magicians' strategy to maintain their oligarchical power, and that there really *wasn't* any difference. But then I remembered that in the first book, Nathaniel was actually taken away from his family because he was found to be a magician.

So what's the deal? How do they decide who's a magician and who isn't? Do they just pick randomly who will be accepted into the upper echelons of society? Or is there actually a gene or something that they can test for?
gpt
There's no "magician gene". Some people are certainly more naturally talented than others, but anyone could theoretically summon demons if they learned the appropriate incantations and rituals. It's just fairly difficult and dangerous. 

The magicians keep magical knowledge to themselves in order to maintain their grip on power. It's a purely artificial divide.