Richard Hughes Gibson at The Hedgehog Review:
What is a paragraph? Consult a writing guide, and you will receive an answer like this: “A paragraph is a group of sentences that develops one central idea.” However solid such a definition appears on the page, it quickly melts in the heat of live instruction, as any writing teacher will tell you. Faced with the task of assembling their own paragraphs, students find nearly every word in the formula problematic. How many sentences belong in the “group?” Somewhere along the way, many were taught that five or six will do. But then out there in the world, they have seen (or heard rumors of) bulkier and slimmer specimens, some spilling over pages, some consisting of a single sentence. And how does one go about “developing” a central idea? Is there a magic number of subpoints or citations? Most problematic of all is the notion of the main “idea” itself. What qualifies? Facts? Propositions? Your ideas? Someone else’s?