Everybody has played the game of telephone where successive attempts to pass a statement between person to person become muddled and increasingly more inaccurate until the final answer has little if any resemblance to what was initially said.
I can only imagine that because all of history until very recently aside from archeological remains could only have been passed down by either the spoken or written word, it being extremely likely that everything we "know" past a certain point must be mostly if not entirely completely wrong.
How can we be certain of Alexander's conquests or Caesars triumph in Gaul actually ever happened as it says it did, just as an example?
And even if we have first hand accounts, such as Caesars memoirs, which are likely filled with biases, fabrications, and embellishments to begin with. How do we know these are accurate if what remains of it today has been written, re-written, translated, and re-translated so many times over?
Can we ever be "certain" of our history past a certain point?
This question, nearly exactly came up barely a week ago with a good answer from u/itsallfolklore
https://www.reddit.com/r/AskHistorians/comments/ttqg14/how_do_we_know_that_our_history_is_correct/
In that same thread I replied with an older answer from a similar question answered by u/DanKensington here:
The long and short of it is that modern historians don't just naively take old primary sources at face value. People doing that is the domain of amateur pop historians and overzealous well-intentioned wikipedia editors.
We don't "know" anything by your definition. Do you know your birthday? How, you sure don't remember? Just your parents told you and you have some documentation to that effect. Why do you trust their memories and that documentation? Ok then, you are VERY confident your birthday is on a date because the documentation seems reliable, and your parents tell you a date. What if your parents told you a date all your life then you looked at your BC and saw a different one? Someone's wrong! Now your confidence goes down. That's the reality - history is about putting together lots of evidence that sometimes corroborates and sometimes contradicts, and building confidence, but not certainty, in certain events. As you can imagine people like Alexander and Caesar generated huge amounts of evidence, not just direct but indirect, not just written but archaeological, not just biased writings but administrative documents, and even then we still don't know all the exact details.
A lot of people can actually get frustrated at how seemingly noncommittal historians are. Almost nothing is a nice clean narrative. Was Xerxes' invasion of Greece a crushing defeat to a massive imperial power by a team of underdog Greeks fighting for freedom....or was it a modestly successful bit of Persian adventurism cut short by a stout defense from the wealthy and powerful in their own right city states? Or somewhere in between?
There's a solid section in the FAQ that goes over some of this.
Just to add on to what others have said ITT, other threads, and the FAQs:
I like to think of it like a crime investigation. If you're relying on just one eyewitness report, your case is probably going to get thrown out. But as you add more eyewitness accounts, forensics (archaeology), documents, and so on all pointing in the same direction, your case gets stronger and stronger. And like a criminal investigation, it's pretty rare for something to be a 100% certainty. That's why the goal is "beyond reasonable doubt."
I've also heard it compared to a mosaic or jigsaw puzzle. You can't just look at one piece and see much of anything. Even if that piece has an inscription on the back that says, "this is what happened." It's still your job as a historian to put together the rest of the puzzle. The color of some pieces might be completely wrong. A lot of pieces might be missing. But if you know what you're doing and put in the time and effort, you can still get a pretty decent picture of what happened.