It didn’t exist in English before then; Shakespeare used “they” instead of “he or she”—we all do, we always have done, in speaking, in colloquial English. It took the women’s movement to bring it back to English literature. And it is important. Because it’s a crossroads between correctness bullying and the moral use of language. If “he” includes “she” but “she” doesn’t include “he,” a big statement is being made, with huge social and moral implications. But we don’t have to use “he” that way—we’ve got “they.” Why not use it?
