In Heinlein's novel Stranger in a Strange Land, one of the characters, a Fair Witness, is asked what color a nearby house is painted. She replies, "It's white on this side," which is all she's willing to say, because she can't see the other sides. I believe this is the most extreme standard of proof I have ever encountered. There are different standards of proof for different things; the criminal "beyond a reasonable doubt" standard is stricter than the one for civil cases. There is yet another standard for genealogy, the Genealogical Proof Standard.
I remember learning to write a term paper in school: being required to keep a bibliography on 5 x 8 index cards; learning to cite my references, and learning the difference between primary and secondary sources. A secondary source in genealogy is often a book called something like The Upham Genealogy, or, to be accurate: Upham genealogy; the descendants of John Upham, of Massachusetts, who came from England in 1635, and lived in Weymouth and Malden. This book was published in 1892, so it's not as if the author had first-hand knowledge of exactly what happened in Massachusetts in 1635.
Primary sources are things that were created for purposes other than genealogy: vital records (birth, marriage, death), census records, church records such as baptisms, cemetery records, and so forth. While these are certainly more reliable than genealogies that trace your ancestry to some illustrious historical person on the basis of a similar surname, they may not be entirely accurate either.
Using last week's example, Jesus would have been listed on the Roman census as the son of Joseph. I don't think anyone wanted to tell a Roman official that the baby was the son of God--or to answer the next obvious question (for a Roman): "which god?" But it does illustrate the weakness of a census: the data is only as good as what the person supplying it provides. During the 1960 census the enumerator did our street on a day when one of the families wasn't home. The man was desperate enough to question a group of us who were playing in the street. We played with the family's daughter, so we did know what the inside of the house looked like, but I don't remember what we said for occupation of head of household. When the census comes out in 2022, I'll have to look and see, but I bet we got it wrong. He was, among other things, one of the two inventors listed on a patent for "Method for improving the in-vivo strength of polyglycolic acid." I'm not sure I understand that now, and I certainly didn't when I was seven. All I could say then was what I knew, which was limited by what I could understand.
"What is truth?" was a good question 2000 years ago, and it's still a good one today.