Let's talk agout unity using a paragraph as an example, although the same comments will apply to essays. Paragraphs are containers for information. A writer must sort his evidence and comments, putting all the information about a particular topic together in the same container. The topic sentence serves as a label for the container, and when the reader looks at the paragrph, he expects to find information about the topic named in the topic sentence. When information on other topics is mixed in, it causes confusion. Therefore, the writer should keep his eye on the topic sentence and not introduce irrevelant thoughts or allow himself to drift away from his topic.
What should a writer do when he discovers that his paragraph is not unified? There are two choices. The first is to delete the information that does not relate to the topic sentence. The second is to find a more general topic sentence that will logically include all the information. The decision as to whether or not a topic sentence logically incudes all the information can be demonstrated through a dependency analysis of the paragraph.
Topic sentences are general statements relative to the information that follows in the paragraph. The general nature of the topic sentence is often signaled by an abstract term. An abstract term is like a pinata; when broken apart, it reveals a vast and often varied content. All of this content is more specific than the "mother term" that enclosed it, and in spite of apparent diversity, it is relevant if it can be logically deduced from the idea expressed in the topic sentence.
You can see how this work in a paragraph from The Dragons of Eden by Carl Sagan:
Some forms of gestual symbolic language, of course, originated much earlier than the primates; canines and many other mammals who form dominance hierachies may indicate submission by averting the eyes or baring the neck. We have mentioned other submissive rituals in primates such as macaques. The human greetings of bow, nod and curtsy may have a similar origin. Many animals seem to signal friendship by biting, but not hard enough to hurt, as if to say, "I am able to bite you but choose not to do so." The raising of the right hand as a symbol of greeting among humans has precisely the same significance: "I could attack you with a weapon but choose not to wield one." (100)
In this paragraph, the abstract term is "gestual symbolic language." This is the mother term that Sagan breaks open to provide specific examples: "averting the eyes and baring the neck"; "bow, nod and curtsy"; "biting, but not hard enough to hurt"; "raising of the right hand."
Here is another example from a book on English history:
One excellent thing abou the guilds was the great care which many guilds took to make sure that only good work would be done. From time to time the officers of the guild searched the shops of members, and destroyed all badly made wares. In the case of the food trades, punishment was usually given by the town magistrates as well. As a rule the bad bread or meat was hung around the seller's neck and he was stood in the pillory for everyone to pelt. Or sometimes he was dragged round the town on a hurdle. Guildsmen who persisted in turning out shoddy wares were usually expelled from the guild altogether, which meant that they could not practice their trade in the town. Night work was forbidden in many places, for at night the guild officers could not search, and by the flickering light of candles, good craftsmanship was impossible. Nor was a guildsman allowd to work in an attic or a cellar; he had to stay where he could be seen --"in halls and shops next the road in sight of the people." (Williams-Ellis and Fisher 132)
In this paragraph, the abstract term "care" is specificed by the examples that show the steps that the guilds took to assure the quality of the products produced by their members. The logical relations among the ideas are revealed by a dependency analysis of the paragraph.