I came across this article the other day and realized it is a good introduction to a very complex subject.
One of the mistakes newbies often make, if they haven't had formal, academic training in history, is to think that an old source is reliable just because it's old. But the opposite is often true.
Our ancestors put a higher value on tradition than on evidence. One example that is familiar to most of us is that people thought the Sun revolves around the Earth -- because Ptolemy said so. They knew there was evidence Ptolemy was wrong but the authority of Ptolemy was considered to outweigh the evidence. Ptolemy simply couldn't be wrong about something like that.
In the same way, if someone a long time ago said something about genealogy then people thought it must be true, even if it was illogical, unlikely, or unprovable. Nowadays we've changed our way of doing things. Good genealogists reject those old myths unless they can be proven from contemporary sources.
So, here's the article:
The Concept of Facts Is Newer Than You Think, by David Wootton / History Today
http://time.com/4671936/history-of-facts/
Let's talk about it. Is this idea new for you? Did you already guess there was something like this? Or have you known it ever since you can remember?