I like the “headology,” or the idea that what people believe is what is real.

“So people see you coming in the hat and the cloak and they know you’re a witch and that’s why your magic works?” said Esk.

“That’s right,” said Granny. “It’s called headology.” She tapped her silver hair, which was drawn into a tight bun that could crack rocks.

“But it’s not real!” Esk protested. “That’s not magic, it’s—it’s—”

“Listen,” said Granny, “If you give someone a bottle of red jollop for their wind it may work, right, but if you want it to work for sure then you let their mind make it work for them. Tell ’em it’s moonbeams bottled in fairy wine or something. Mumble over it a bit. It’s the same with cursing.”

– Equal Rites, Terry Pratchett