Let's see. English peasants after the Norman Invasion dropped Old English and started chatting in Old Norwegian, but being peasants and not being able to afford Rosetta, they didn't pick it up properly, but added a bunch of Old English-derived vocabulary because it was sitting around unused in a trunk. Leave those peasants alone with their badly spoken Old Norwegian and viola, Middle English. Sure, OK: makes as much sense as the that-which-restrictive-non-restrictive-clause rule.

Ceci n'est pas un seing.