The unit of composition
Famously, Strunk and White refer to the paragraph as "the unit of composition". Remember from our discussion of sentences, we suggested that sentences express simple ideas. Paragraphs, being collections of sentences, must express complex ideas. We may refine this by saying that a complex idea is a supportable idea.
Let's pause here to think about this. This suggests that there are (at least!) two kinds of ideas. There are simple ideas, which we express in sentences. And there are complex ideas, which we express in paragraphs. Now, it might be that we can give further reasons to think a simple idea is true -- by definitions or observations or some such thing. For example, we can say that "red is a primary colour" is a simple idea, expressed in a sentence. For most purposes, we don't find a need to say more about what the sentence expresses. However, we could, in some exotic circumstance, say that red is a primary colour because no combination of two other colours produces it. As always, context dictates the terms.
(This point drifts uncomfortably toward deep and abstract philosophical studies, so we'll leave it at that and move on. But note that if you're interested in the nature of simple ideas and complex ideas, there's much more to say on the topic.)
Typically, simple ideas are not particularly interesting ideas; they're basic facts from which we build up the more interesting ideas. This is, at least partly, why the paragraph is the unit of composition rather than the sentence. Our compositions are centred on interesting ideas, and complex ideas are the interesting ones worth writing about. Hence, the paragraph.
To put it even more simply:
A paragraph is an idea with reasons.
The simple ideas in sentences are reasons to think the more complex idea of a paragraph is a worthwhile idea to entertain. And buried in this point is the fundamental idea that:
A paragraph contains only one complex idea.
Or at least it should.
Typically, a paragraph starts with a sentence suggesting the topic. Then, the sentences that follow provide support, in one way or another, of that topic. A final sentence may be a restatement or clarification of the paragraph's topic. In any case:
A paragraph opens and closes.
The sentences between the opening and the closing connect the two points. Any sentences that do not advance this end simply don't belong in the paragraph.
Notice that if you treat the paragraph as the basic unit of composition, and you excise the simple ideas that are not part of the the complex idea the paragraph advances, your writing will tighten. This is an especially good goal for non-fiction writing, and especially any sort of writing that aims to persuade a reader of a point.
Dwell on this final thought for a moment. "A paragraph opens and closes." This suggests a tight, well-defined structure; a paragraph is not whimsical and does not ramble. A paragraph serves a purpose and you should have a clear plan for how to introduce that purpose, and then how to show that purpose met before moving on to the next complex idea in your chain of reasoning. (This foreshadows our later discussion on Structure, where we will again consider opening and closing, but in terms of a whole essay.)
In the reading and activity to follow, consider what Strunk and White meant about the paragraph: what does it mean to be "the unit of composition"? You'll be asked to write a descriptive paragraph about yourself, which is a good opportunity to include a few lessons in one spot: consider whether you'll describe yourself through actions or through adjectives. Feel free to discuss these considerations in the forum at the end of this module.