Some things are hard to believe. In school we sometimes mistake them for things that are hard to understand. We are, in a sense, too trusting, too humble; we imagine that the truth of these things is certain and obvious to others and that we’re just not smart enough to “get” them. But there are many truths that you are right not to believe before you are given good evidence. Your apparent difficulty in understanding is actually the dissonance you feel between your respect for your teachers–your assumption that what they are saying is true–and your entirely reasonable doubts about whether the world is really as they say it is. Consider this possibility while you are learning and while you are reading what others write.
And consider this possibility when imagining your reader as you write. Will your reader find what you are saying hard to believe? Is it reasonable for the reader to doubt that what you are saying is true? This does not mean that you have to doubt yourself. You just have to respect the reader’s doubts and the easiest way to do this to recall to yourself the process by which you came to believe what you believe. When the idea first occurred to you (perhaps by being presented to you in someone else’s writing), did you immediately believe it? Or did your belief come only after you discovered (or were shown) a body of evidence to support it? This evidence is what you, too, need to present your readers with if you want to help them overcome their difficulty here. Don’t immediately imagine that they don’t understand it (we’ll get to this difficulty next). Imagine that they just find it a little implausible.
Many things that are worth knowing can only be known after long struggle. This struggle produces the evidence to justify the belief that is a necessary condition for knowing anything. (Leaving aside some philosophical technicalities, you can’t know something you don’t believe.) If we just believed everything we were told, we could avoid this struggle, but we would also not know anything. After all, we are told things that contradict each other. We literally can’t believe everything we are told because we would end up contradicting ourselves. So we do well to maintain a particular standard. We want to see the sources that others have used to arrive their beliefs. Then we make up our minds.
Like I say, to acknowledge that the reader might find what you are saying hard to believe does not require you to be in a particular state of doubt. It does, however, require you to acknowledge that you might be wrong. There are a bunch of facts that you think obtain, but you’ll grant that if they do not this undermines your basis for believing the thing you are trying to convince your reader is true. Those are the facts you present to the reader as evidence for what you are trying to say. If they will grant you those truths they will also grant your main point. That’s how it works.
In your paragraph, then, you articulate your main point in a good, strong key sentence. It should simply declare the belief that you wanted to convey to your reader. Since you know the reader will find it hard to believe you should make it easy to understand. Make it very clear what would be the case if it were true, fully recognizing that the reader, somewhat surprised at the image, won’t immediately grant that it is. Now, in implicit acknowledgement of the reader’s skepticism, the reader’s difficulty believing you as you imagine this difficulty in your reader’s mind, present a number of facts that are each easier to believe than your main point and together support it. By the end of the paragraph, with these facts properly organized around your main idea, the reader should find it easier to believe.
No argument is perfect, you are just trying to help. You are trying to help the reader overcome the difficulty. Good writing should always make things easier for the reader. Believing, however, is only the first difficulty. Stay tuned for Difficulty #2.