There's a concept which has given me a big fat headache for a few days now. But it’s also fascinating.
It’s called "historical disappointment".
The idea is based around practices that used to be culturally acceptable, like slavery, that we now consider offensive. These days we realise how evil slavery is. But if we’d have been brought up in Roman, Egyptian or Viking times, it’s likely we’d have not even questioned the idea that some humans had no rights at all.