One of the easiest ways to spot when a historian does not know something is to look for the language they use to try to hide that fact. For example, the sentence “Undoubtedly, George Washington was angry when he got the letter” actually means “I have no idea whatsoever how Washington felt, but I’m going with ‘mad.’” Undoubtedly is one of the most common ways historian admit ignorance, but they have many similar stock phrases, all of which basically boil down to “this is my guess.” The fact is, though, that it is hard to know stuff. Any historian worth his or her salt will be painfully aware of all the little (or not so little) gaps of knowledge in anything they write. Sometimes the line beyond the gaps goes pretty straight, so it is not too hard to leap the gap and still be on the right path. But sometimes you just fall into the gap.