Specifics: the magical ability is genetic, very rare (about one in ten thousand people will be born with it), and no mage can be too strong (no destroying mountains, drying lakes etc.). Also, the nature of magical spells is fundamentally different from any technology or natural phenomena (i.e. laser beams or lightning), even if the effects can be similar. In other words, think Harry Potter kind of magic (not literally, but close enough).
But there is a third group of people, who have a genetic ability to resist any magical effect (be it physical or mental). Their numbers are approximately the same as for people with magic. In other aspects, they are normal, with no special abilities.
Let's call the groups mundanes, mages and insulars (the third group).
Some of the effects:
- Mages would find it very hard to harm insulars with magic (some ways are still possible - for example, setting fire to a house would kill an insular if the magical fire causes an ordinary one. Dropping a large stone on them should work as well);
- Mages would find it hard (almost impossible) to help insulars with magic. For example, no magical healing would work on them, be it spells or potions; no teleportation, etc;
- Insulars would probably benefit more from living in mundane society, however the mage society wouldn't be able to hide from them with mental magic - they will see through any such disguise. So to hide from insulars, mages would have to completely separate themselves from the mundane society.
- Mundanes who know and fear magic would either want to employ insulars to search for and fight mages, or hate and fear them as well, becasue insulars are better protected.
Question: In a modern setting with total population about the same size and the same level of technology/medicine, how would the three groups coexist realistically?
We assume that around Middle Ages everyone knew about magic, or at least suspected.