I'm encountering a similar principle in genealogy. A lot of amateurs make the mistake of assuming their beliefs about ancestors are facts, and build a family tree based on their assumptions. But each linkage in the tree is vital. The chain (of ancestry) is only as strong as its weakest link. If Joe didn't really come from Mary and Bill, then even if you're related to Joe, you aren't (closely) related to Mary or Bill... or any of their ancestors. Some people build a family tree of cards, taking beliefs as truths, and forming a false view of who/where/how they came from. My strategy, then, is to start from myself and build backward, to my parents, and their parents, and THEIR parents, etc, but only proceeding back in time if I have documents and/or photos and/or personal memories and/or reliable firsthand info from relatives.

A professional genealogist I met recently told me "in genealogy, information is either properly sourced, or it is mythology". Assumptions are beliefs, and even though they might be true, building upon them is foolish.