Aliens

Yes. Unless we see it ourself, we're going to make some kind of mistake about it.

I think that this claim is not self-evident in general. It seems dependent on the situation, rather than a generality. Surely you can think of claims that you can not observe directly, but can be reasonably certain about.

Sorta like when the word "slave" sets off a Black guy... even though, he, and everyone he's ever met, was never a slave.

Do you not think that the repercussions of ones history is a legitimate cause for an emotional response; in some cases, anger? And isn't a grave social injustice, no matter when it happened in history, worth being angry about, especially if someone suggests that your entire race should once again be subjugated to it?