Unmotivated topic shift
Texts have both a hierarchical and linear structure. Entities (objects, topics) are introduced, and then developed. Since objects can be viewed from many perspectives, it is important to signal the viewpoint. The first entity is generally the perspective from which the described topic or scene is viewed. Topics are generally confined to a paragraph, and unless being “broadcasted”, they remain stable. Consider the following paragraph from one of O’Connor’s novels [OCO 71]
“Mrs. Shortley was watching a black car turn through the gate from the highway. Over by the toolshed, about fifteen feet away, the two Negroes, Astor and Sulk, had stopped work to watch. They were hidden by a mulberry tree but Mrs. Shortley knew they were there.”
If in this last sentence the author had used the active voice, writing, “The mulberry tree hid them but...”, the text would still be correct, though much less fluent. Indeed, the use of this particular grammatical option would introduce an unmotivated topic shift2, changing the perspective from “Astor and Sulk” to the mulberry tree.