Unity is the Anchor

Unity is the anchor of good writing. So, first, get your unities straight. Unity not only keeps the reader from straggling off in all directions; it satisfies your readers’ subconscious need for order and reassures them that al is well at the helm. Therefore choose from among the many variables and stick to your choices…One choice is unity of pronoun. Are you going to write in the first person, as a participant, or in the third person, as an observer? Or even second person,…Unity of tense is another choice Most people write mainly in the past tense…Another choice is unity of mood.

Zinsser, p. 50.