Way back in the late '90s I read a book that I've been trying to remember the title of for damn near a decade. The magic in the series was done with ranked stones, the possession of which was supposed to unlock magical power.
Later on in the book it's revealed that it's actually the focusing of the human mind that allows for magic, and the stones merely assist in that task, when the main character manages a minor spell to help in in a sword fight. This is further reinforced when they find a town of people with a mental disability of undisclosed origin, where the birds were unable to fly. It was explained that the leader of these people had declared that they couldn't, and their mental state allowed them to both believe that and focus on it collectively, strongly enough that it evoked the effect.