I remember being kinda frustrated in the first few grades of school over how they couldn’t give a rule for when to break paragraphs. For sentences, the capital letter at the start, period at the end was easy enough (although what a sentence is isn’t entirely clear even to linguists xbar mp s np vp foo bar frotz), but for paragraphs they couldn’t say anything. It was more a “know it when I see it”; great, what does that help us?
Now that I know a li’l bit more, I think one reason for why they couldn’t is because sentence division is on the syntax layer while paragraph division is on the semantic layer. My guideline is that a paragraph should start and end “being about the same thing”, unlike a DJ segueing from talking about one song to another. Introduce a new concept? New paragraph.
Of course, since the world is endlessly fractal and metonymic, where everything could be seen as a subtopic or sibling topic of something else (“whaddayamean ‘new things’? A banana and a politicians are both from the same planet!”) so that still leaves a lot of freedom in how to split paragraphs. Got a lot of things to say about one thing? Then maybe the things you’ve got to say are things on their own, in paragraphs of their own.
For decades I really relied on making lists and outlines instead of prose; I loved laying out an argument in list form, that was really how I thought, too. So instead of walls of text, I’d post 1, 2, 3… or A, B, C…, like “here’s a list of problems with this economic theory”.
Moving away from list-making wasn’t a conscious decision, it wasn’t something I decided to train myself away from. I just gradually got more comfortable with expressing the same ideas using paragraphs and headlines. It’s still lists under the hood.