When It's A Jar

Maurice has just killed a dragon with a bread knife. And had his destiny foretold... and had his true love spirited away. That's precisely the sort of stuff that'd bring out the latent heroism in anyone. Unfortunately, Maurice is pretty sure he hasn't got any latent heroism.
Meanwhile, a man wakes up in a jar in a different kind of pickle (figuratively speaking). He can't get out, of course, but neither can he remember his name, or what gravity is, or what those things on the ends of his legs are called... and every time he starts working it all out, someone makes him forget again. Forget everything.
Only one thing might help him. The answer to the most baffling question of all...
WHEN IS A DOOR NOT A DOOR?