[For the previous post in the series, see here]

“Words are to sentences what atoms are to molecules: the basic building blocks that control structure and function. If we extend that analogy, paragraphs become cells: the fundamental unit of life. A cell gains life from its structure, a structure that creates internal cohesion and external connection, allowing it to function as part of a larger organism.” —Joshua Schimel, Writing Science

Finally, after all this planning and outlining, it is time to start filling in the blanks. It is time to write words that form sentences that form paragraphs that form sections that form your story.

Because of the top-down approach that has brought you here, coming up with words and sentences should be easier than having to start with a blank page. You should already have an outline of the paper as well as notes for each paragraph—now you only need to turn those notes into full sentences!

From the point of view of the reader, the best sentences are those that are easy to understand, that make the story flow, that tell the right things in the right order, so that the reader can always expect what comes next. From the point of view of the writer, this is achieved if the writer feels empathy for the reader. A good writer tries to look through the reader’s eyes, taking the reader’s hand and guiding her through the text, making the reader’s job as easy as possible.

To guide the reader through the text, the writer has to gently manipulate the short-term memory of the reader. It has been argued that one’s short-term memory can only hold seven things—such as digits—at a given time. When it comes to concepts that are more complex than digits, even seven sounds like an awful lot to me. There is only so much one can hold in his head.

When a piece of text feels too hard to grasp, this is often not only because the ideas therein are difficult. Instead, there may be a problem with the sentences that should deliver those ideas. This has to do with the order in which the concepts that the sentences contain are placed into the short-term memory of the reader. Bad writing randomly jumps from one thought to another. This creates a traffic jam of thoughts and ideas without giving a clue as to how they relate to one another.

What, then, is the right order of things? How to choose what comes first in a paragraph or in a sentence? To answer this question, we have to understand how the reader processes information—how the reader reads. To this end, let’s construct a toy model of the reader—think of the reader as an automaton of sorts, with three types of memories that you can manipulate. Each type of memory plays a role in interpreting and understanding the paragraphs and sentences that the reader encounters.

The first, long-term memory contains the key concepts that are required for understanding the paper. Using the film script analogy, these are, if you like, the world of your story and its inhabitants. This memory is initialized in the Introduction, and populated with more characters and events as the reader reads. Whenever new concepts become established in the paper, they are added to this memory.

The second, intermediate-term memory contains concepts essential for connecting the dots currently in front of the reader—for making sense of things in the current paragraph and in the current sentence. In particular, this memory contains the topic of the current paragraph (or the reader’s interpretation of it). Not all concepts remain in this memory for long: many are flushed out at the end of each paragraph. If you know time series analysis, you can think of this memory as a sliding window of sorts.

The third, short-term memory holds the concepts that are required for making sense of the current sentence, as in “what is the subject of the sentence, what is the verb, what is the object”, or, “what does what to what”. In addition to grammar, the short-term memory is essential for relating the concepts encountered in the sentence to one another. As the reader reads, the words and concepts of a sentence are placed in this memory in their order of appearance. This memory is flushed at the end of each sentence.

How do these memories work? When the reader reads, she consumes the words from the left to the right and interprets them with the help of all the memories. The long-term and intermediate-term memory help to extract the meaning in the sentences. They also and set the reader’s expectations, “bias the reader” if you will, and determine how words and concepts are interpreted. The short-term memory is used to parse the current sentence and to connect its words to one another.

If all goes well and the reader understands the words in front of her, the concepts that those words hold are first put into the short-term memory, to be flushed out at the end of the sentence. The higher-level concepts formed by the words in the short-term memory are placed in the intermediate-term memory, to be used in deciphering the rest of the paragraph. At the end of each paragraph, this intermediate-term memory is given a vigorous shaking so that those concepts that are unnecessary baggage fall out, never to be seen again. But those that stay and prove to be useful may transcend to the first, long-term memory.

But if the reader encounters something that is inexplicable—something that doesn’t match with anything in any memory stack—an error occurs, and the reader is lost. Or, worse, if the reader uses a wrong concept to parse the current sentence, she can become entirely derailed.

Because of the way this parsing automaton operates, the first words of a sentence are tremendously important, as is the first sentence of a paragraph.

The first words of a sentence initialize the short-term memory, determining how the rest of the sentence is understood—they define the topic of the sentence. Then, when the reader reads on, the words that she encounters are seen interpreted through the lens of the topic. As more and more words accumulate in the memory, the reader is trying to tie them together and make sense of the sentence. The more words there are, the more difficult this gets (think combinatorial explosion). In particular, if the sentence is constructed such that its meaning hinges on the last word(s), it becomes taxing to read.

So, from the point of view of our short-term memory model, the perfect sentence begins with words that clearly define its topic, and is not too long. Any superfluous words like “very” wherever it appears or the “in order” in “in order to” just make the reader’s job more difficult by increasing the number of possible combinations. Also, it helps if the order of words allows one to parse the sentence up to the current word, so do keep the subject and verb close. And one more thing: place familiar words and concepts at the beginning of the sentence, and use them to explain any words or concepts that might be unfamiliar to the reader that appear later on in the sentence.

When it comes to paragraphs, the first sentence of a paragraph establishes its topic, and this topic is stored the intermediate-term memory for interpreting the rest of the of the paragraph. So always make sure that the first sentence of each paragraph is well-chosen and clearly tells the reader what the paragraph is about. This sentence sets the expectations of the reader and determines how the reader will attempt to interpret everything that follows. Always stick to the topic in the rest of the paragraph—an unexpected, unconnected sentence breaks the flow and leaves the reader baffled. But a sentence that makes sense in light of the topic will be interpreted properly, setting expectations for the next sentence as well.

One way to make the topic of the paragraph clear is to begin with a phrase that acts as a signpost that clearly tells the reader where the paragraph is going. Examples include “To measure how X depends on Y, we constructed an elaborate apparatus…” and “In conclusion, in this paper, we have shown that…” When the reader knows what to expect, interpreting the rest of the paragraph is easier.

But not only beginnings are important—endings matter, too, just like for stories. Because the end of a paragraph and the end of a sentence signal a break, the reader has more time to think about the last few words. Therefore, the last words carry extra weight in the mind of the reader, almost unconsciously.  Place important things in these stress positions, things that you want to emphasize.

Next: how to write your first draft.