[throwmeaway222]

I think he's making fun of historians that have really dumb reasons for declaring human culture is one way or another because of event X but it doesn't pass a sniff test. In this example, mirrors don't need to exist because people can look at other people (or more simply feel their own face). It was a 40% funny joke.

[mjd]

Not a bad guess! According to this report on my desk, it is 61%±2% funny.

[throwmeaway222]

I reran the numbers and it did come in at 59.5%

[djmips]

don't forget water and maybe oiled polished obsidian?