I am interested in this question: how do the historians prove somebody really exist, and he really did something?
Let's use Aristotle and his Politica as example. We say 'Aristotle wrote Politica thousands years ago', but isn't it also possible that Renaissance scholars phony this book and added their opinions to it? Because anyway our current knowledge about ancient Greek people and books are from quotes by the scholars. Arabic scholars collected and translated books, and Renaissance scholars found them and re-introduced them to European. Among all these processes and long time periods, many things can go wrong. Three could be translation error, there could be words distortion, adding or deleting...
Although I don't believe the latter one, I can't find a very strong argument to support the first statement.
We can't say 'because the ancient people said so, therefore it is true' because ancient people may lie and their words could also be distorted by the quoter. If the archeology finds some old scripts, it only can prove that some parts of the book is old enough, but not necessary 100 percent belongs to Aristotle. It is still possible that many writers contribute individual chapters or opinions, and someone late has combined all the thoughts to one book and put Aristotle's name at the beginning of the book.
So what other strong evidences do we have to prove those famous books, from philosophy to physics to politics to logic..., were really wrote by Aristotle and what we read today are consistence with what he wrote at that time, rather than it is faked or gathered by someone else? Or do we simply 'believe' it because the other possibilities are too unlikely and too conspiracy?
I believe this is actually a philosophy question, not a historical question. i.e. "how can we prove anything exists?"
I'm not a specialist in philosophy, but I think this was the core concern Rene Descartes attempted to demonstrate with his "I think therefore I am." At the extent to which you radically doubt everything, the only thing you can prove is your own existence by the fact that you have questioned it. However as any good solipsist will tell you, that doesn't prove that anyone else exists.
History, like many fields, depends upon an agreement upon "what considerations we set aside." We presume that a past exists, and that writings from the past reflect attitudes at the moment of its writing.
Empirical science depends upon agreeing that inductive observation works and that repetition of something in the past most likely guarantees repetition in the future. "I've seen this rock fall on the ground 99 times before, so it should fall the next time I drop it." But what if instead it magically floats in the air?
These are, of course, not historical method questions. They are philosophy questions. Which is why you might want to try this question in /r/askphilosophy for a better answer.