How we know that what we know is the actual truth? Let me bring the example, very basic, you say "she is a girl". How do you know that? The question seems very silly and you probably thinking now that I am insane (and you are not really wrong), but let me ask you "how do you know that what you said is right?"
You now look at me in a weird way and you say "because that is what I been taught." Fair enough. Now is the big question. Wait for it...
How do you know if what someone did teach you wasn't wrong? How do you know that the people that did teach you were not taught wrong?
Alright, actually there were two questions. I was always bad at maths.
What if a girl is actually a boy and the boy is actually a girl? Confusing right? Who knows if someone didn't made a mistake in categorising people by gender and then next generations didn't picked up that mistake? People are only humans, so they could make a mistake. But we just want to believe that what we are being taught is right. Because that is simpler.