If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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Here's a fun game: concepts, ideas, institutions and features of the world we (let's say 21st Century Westerners) think of as obvious, but aren't necessarily so. Extra points for particularly visceral or captivating cases.
For example: at some point in human history, the idea of a false identity or alias wouldn't have even made sense, because everyone you met would be either known to you or a novel outsider. These days, anyone familiar with, say, Batman, understands the concept of an assumed identity, it's that endemic in our culture. But there presumably must have been a time when you would have had to go to great lengths to explain to someone what an assumed identity was.
Given these examples, it might be interesting to add to this thread with examples of ideas assumed to be new that are in fact old.