I do like nerding out on language, yes, and I feel I’m in good company now. ;) Right, so independent as a term of grammatical art (or ‘improper noun’ as suggested by an article posted to HN once https://news.ycombinator.com/item?id=32673100) suggests that you can simply replace a semicolon with a period and get two grammatically valid sentences. That’s basically what rule 6.56 is saying. It’s possible I exaggerated a tiny bit, and commas aren’t literally always swappable for semicolons in every single case. I’d concede that professional writing has stricter limits. But I do believe semicolon in literature and poetry and casual writing is almost interchangeable with a comma, and it’s far more a matter of degree and style than of correctness. Outside of publishing where an editor is involved, I would argue the vast majority of writers and readers use and interpret semicolons as emphatic commas, and I would argue that written language is descriptive anyway, despite any claims to the contrary. Only constructed languages are prescriptive, all others are formed through usage.

Anyway, yes you’re right, when I suggested ‘therefore’, I was implicitly suggesting 6.57’s “; therefore”. I wouldn’t be at all surprised if a style guide said that starting a sentence with “That’s” is to be avoided, but as far as I’m concerned it’d be in the same bucket as starting a sentence with “And” or “But”: a rule just begging to be broken as often as possible, in beautiful ways. Pretty sure that’s how poets treat the semicolon already. :P

The problems with style guides are they don’t represent most writing, and they often represent publishing fads, and they change. Things that were in the style guide when I was a kid aren’t the same anymore. They try to be prescriptive, but that doesn’t work long term. Reread Google’s definition; I think it’s very intentionally vague and devoid of tricky rules. It almost says a semicolon is a double comma. And it’s probably used (I speculate) a lot more often than M-W, Chicago, or Wikipedia or any good source we can find. Also be sensitive to whether Chicago or M-W or anyone else talks about what you can’t do with a semicolon, as opposed to what you can do. It’s not an accident that they mostly avoid drawing hard negative bounds.

One of my college English professors taught the class to think of a semicolon as a mid-sentence question mark where the answer is immediately provided in the remainder of the sentence. I’m kicking myself for not grilling my grandfather about all things English before he passed. He taught English, specializing in Shakespeare and poetry.

Part of my attitude just comes from other examples where there are fads to nitpick language, and these fads are often wildly popular and yet completely and totally incorrect, historically. People who harp on the figurative use of the word ‘literally’ are incorrect. People who claim that ‘less’ must be used instead of ‘fewer’ are incorrect. ‘Myriad’ can be followed by ‘of’; ‘really’ can be used as emphatic rhetoric; sentences can be separated by either one or two spaces; and semicolons are used in a very wide variety of ways. There was a great piece on This American Life about vocal fry and how so many people become sticklers only after learning the term. This seems to be what some people do with grammar - learn something and then want to show off their knowledge by policing others without realizing they only have a tiny slice and don’t know the whole story. Language is necessarily vast and changing and fluid in super interesting ways!