Qualities of expression: Unity of substance

in-nuce.com: Unity of substance
"Mrs. Bates was very chatty and good-humoured."
Illustration by Hugh Thompson for Jane Austen's Emma.
Source: solitary-elegance.com

A sentence that contains more than one leading thought is not a unit in substance.

From Beginnings of Rhetoric and Composition, by Adams Hill:

Unity of substance

Too Much in a Sentence

A skillful writer might put all that needs to be said into a single complex sentence. An inexperienced writer, however, is more likely to secure unity if he does not try to get too much into one sentence
1. Put ideas that are only loosely associated into separate sentences:
John possessed a small amount of book-learning, but had seen little of the world. He was conceited and arrogant, and prone to temper tantrums.
The connection of John's book-learning and ignorance of the world with the traits of his personal character is so slight that there is no sufficient reason for putting both sets of ideas into one sentence.
2. Put general characteristics into one sentence, and how they are manifested into another:
Swift's ways were coarse and vulgar. He would irritate a man by making fun of him, just for the pleasure of putting him under his feet.
We promote unity by separating Swift's characteristics from their manifestation.
3. Put something someone says into one sentence, and their reply into another
The shepherd promised to bring Rosalind to Orlando the next day. Orlando replied that if this were done — and he doubted very much whether it would he — he was ready to marry Rosalind if she were willing.
We promote unity by giving one sentence to the shepherd's promise, another to Orlando's reply.
4. Don't attempt to tell a long story in one breath. For example:
I was reading it to Mrs. Cole, and, since she went away, I was reading it again to my mother, for it is such a pleasure to her — a letter from Jane — that she can never hear it often enough ; so I knew it could not be far off, and here it is, only just under my huswife, —- and since you are so kind as to wish to hear what she says; but, first of all, I really must, in justice to Jane, apologize for her writing so short a letter, only two pages you see, hardly two, and in general she fills the whole paper and crosses half.
This long sentence lacks unity in every respect and from every point of view. It fell from the lips of Miss Bates, a character in Jane Austen's Emma who is as slipshod in mind as she is tedious and confusing in speech. 
Such a sentence, by trying to say everything at once, says nothing clearly or effectively: it violates unity of time, of place, of subject, one and all.

Too Little in a Sentence

5. It is also an offense against unity to scatter through two or more sentences a thought which belongs in one.
A. Finding in the bed a person whom he supposed to be his brother, he got a pail of water and poured it over the unlucky sleeper.
B. B. He found a person in the bed and supposed it to be his brother. He accordingly got a pail of water and poured it over the unfortunate sleeper.
In this example, the main fact is that "he" poured a pail of water over a person whom he mistook for his brother. In passage B, a part of this fact is in one sentence, a part in another; by putting the two parts into a single sentence, we secure unity.
A. I left my brothers in the library and crept upstairs to the garret, took the ladder that I found there and climbed onto the roof. I stood the ladder against the eaves in order to climb up.
B. I left my brothers in the library and crept upstairs to the garret, took the ladder that I found there, stood it against the eaves, and climbed onto the roof.
Here, in passage A, the subject is already on the roof by the end of the first sentence, rendering the second sentence both redundant and anticlimactic. We secure unity in passage B by putting the complete idea into one sentence.
 


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