People sometimes tell me they have a hard time clearly distinguishing the three rhetorical postures of the paragraph: support, elaborate or defend. I usually correlate them with the three difficulties that your imagined reader might face when reading your key sentence out of the context of the paragraph. Will the reader find it hard to believe, hard to understand, or hard to agree with what you are saying? You should support it with evidence, elaborate your meaning, or defend it against the reader’s likely objections accordingly. Writing is hard, but only in order to make reading easier. You write a whole paragraph because you can’t, as a scholar, merely assert your claim. It is something a bit heavier than that, if you will, or a bit more subtle, or perhaps a little edgy. “Yes, yes,” the writer says, “I heard all that and I get it at an abstract level. But can you give an example?”
Here’s one that sometimes helps. My office, where I’m writing this, is on the third floor of a big, modern university building. Suppose I tell you, “There’s an elephant in the lobby.” Well, you might find that somewhat hard to believe. If so, I could show you some evidence of the elephant in the lobby. I could perhaps take some pictures or cite witness reports or upload a video of the panicked students fleeing the scene. Or I could show you the event poster of the circus holding its career day and say, “I know it sounds strange, but they decided to bring a baby elephant to draw a crowd.” There are lots of different ways to try move you from a state of disbelief about the elephant in the lobby to one of belief. Some of these could easily be carried out in a coherent prose paragraph that supports the key sentence “There is an elephant in the lobby.”
But notice that this only works if we assume you understood what I meant and that you were able to doubt my word. Suppose we invert these priorities; suppose you are predisposed to believing what I tell you. When I say, “There is an elephant in the lobby,” you experience a difficulty, but it’s not the difficulty of believing me. You assume it’s true; you want to believe. But the statement puzzles you. How could there possibly be an elephant in the lobby? What are you not understanding? At this point, I could explain that it is, in fact, a statue of an elephant that has been donated by the Carlsberg Foundation to the business school. (Like the real elephant, I should say, this one is also fictional. We don’t have a statue of an elephant in the lobby.) “Ah! I get it,” you say. Now you understand what I mean and it all makes sense. I took the key sentence and I elaborated on it and the puzzle has been solved, the difficulty, overcome.
Finally, let us suppose you have just walked through the lobby and seen a hippopotamus there. In this case, it doesn’t matter whether it’s a real one or a statue, but lets suppose that it is a piece of modern art that is ambiguous on this point. The important thing, however, is that you have experienced it yourself and have made up your mind, based on the evidence, that it is a hippopotamus. When I say, “There is an elephant in the lobby,” your difficulty is not one of believing or understanding me. You know exactly what I’m talking about but you can’t agree with me. And suppose, further, that I am also aware of your position (or the existence of a position like yours). I know what your arguments in favor of calling it a hippopotamus and not an elephant are. I can now spend my paragraph defending my interpretation of the block of granite against yours.
Keep in mind that a paragraph is only one minute of your reader’s experience. The key sentence should occasion a difficulty in the reader’s mind that can be resolved within that time. You have to be realistic about your ambitions. It’s not that the reader must end up believing you with firm conviction, only that your claim should have become more credible by the end of the paragraph. It’s not that the reader’s mind must be filled with pristine light upon your meaning, only that it should be less puzzling than when you began. Finally, don’t think you’re going to make your reader agree with you with a single paragraph. There are the rare cases when you might know exactly what to tell the reader to change their mind, where you can point out, for example, that the sculpture you’re talking about was, in fact, called “The Elephant” by the artist (and the brass plaque on its base says so). But the normal case will simply be one of managing the disagreement itself, situating it in a larger argument. I hope that helps.
To haters of the five-paragraph essay, please notice that this is a perfectly valid example of one. Why don’t you want to teach students how to make one of these?