How do I know what I know?

I don't know what I know... huh?

How do I even start explaining this?
It is a known fact that the earth is spherical(real word?). That's just the way we see it. But, there may be something up with human eyes that distorts everything we see.
It's like telling a woman/man born blind that a Domo cup is brown. This is may or may not be exclusive to me, but if I was the blind one, I, of course, can't see it. I would understand that that piece of information is a claim. It may be what this person sees, so it's a truth- to him/her. It could be a lie and it's really black.

To me, nothing is certain; everything is a claim because it can be a lie or an untruth. A lot depends on perception and perception is different from person to person, place to place, species to species, so I try to avoid assuming anything.