> Gramatically speaking, none of these are plural. But even you used a plural, by saying "books" in #1, demonstrating that even when coming up with this exampl

> Gramatically speaking, none of these are plural. But even you used a plural, by saying "books" in #1, demonstrating that even when coming up with this example, you did not "resolve" anything just because "everybody" is a singular pronoun. You used "they" not referring to the word, but to what the word logically implied, and thus this "they" is also plural. I simply don't see this natural progression that you claim. I see distinct usages of "they" for two different types of indefiniteness: one of number, and one of sex. #1-#3 show the first type. It's old and uncontroversial. (You seem to place #3 differently, but as written, the speaker cannot know whether the owner of the book and the owner of the desk are identical, or two persons.) But #4-#6 don't exhibit any numerical indefiniteness. Here the "they" is clearly singular. It's my understanding that this usage is novel and did not arise naturally, but is the result of a political campaign, starting about 1960. That's why I asked for older historical examples, which could disprove this theory.