Double negation makes people more inclined to believe the "truth" that is ultimately discovered.
This is why Fu Qian didn't use flame enchantment on the first two strikes after realizing the prisoners had consciousness.
The most perfect scenario would be that the prisoner knew a lot about the pale figure and had enough worldly experience not to be deceived by simple appearances, querying his own disguise from certain details.
After taking a hit next, he would "personally" experience it and successfully verify his suspicion.
Then when struck again with an enchanted blade, it would help him realize there was another cause for the anomaly, overturning his previous conclusion.
Ultimately, the identity of the pale figure and spread of the golden flame would be recognized to the greatest extent and most likely passed on to the Night Watchman.
All seems to have executed perfectly so far.
