What is the most realistic way to add magic to a world based on medieval Europe?

Edit: Thank you for all the responses. I'll take them into consideration.<br><br>So I have been working on a fantasy world based on medieval Europe for some time now, and I've recently decided to add magic to the world. I've tried many approaches to magic systems, but none of them have really seemed to fit. I've looked at other's magic systems, but none of those have been what I'm looking for either. I've tried reading up on magic and witchcraft in medieval Europe, but that hasn't really helped.<br><br>At this point, I've decided the best approach is to ask for help. What is the most realistic way to add magic to a fantasy world based on medieval Europe? Thanks in advance.