My issue with this definition is that it includes deluded charlatans, can be applied to unfalsifiable (unknowable, even) propositions, and depends on the gullib

My issue with this definition is that it includes deluded charlatans, can be applied to unfalsifiable (unknowable, even) propositions, and depends on the gullibility and cognitive biases of the general populace. So for example, Jesus "knew" that he was the son of God, even though a more rational interpretation is that he was mistaken in his own belief but charismatic enough to convince many others. (Please replace Jesus for another religion's prophet if you are Christian!) Also I don't think this definition fits with people's intuition. At least, certainly not my own. There are times where I realise I'm wrong about something I thought I knew. When I look back, I don't say "I knew this, and I was wrong". I say "I thought I knew this, but I didn't actually know it".