These aren't the sort of contradictions you care about in a literary or role-playing fantasy world. Players typically cut you a lot of slack when it comes to ec

These aren't the sort of contradictions you care about in a literary or role-playing fantasy world. Players typically cut you a lot of slack when it comes to economics and ecology. But they do care about what you've said. If you told your players that all magic comes from the sun, those creatures from the deepest caverns better not have any magic. You can make up excuses as you go along, but they will think you a worse GM for it. If they draw an inference that was valid from what you'd said, ("Hagrid said every single wizard who had gone bad was in Slytherin. At the time he said it, everyone thought Sirius Black had gone very bad indeed. Ergo, Sirius Black must have been in Slytherin! See, they can be good!"), that's annoying in a book, but in a role playing game it's downright frustrating.