COMMAS AND PARENTHESES
The differentiation of the comma, or commas, from parentheses, is the differentiation of the purely parenthetical from the slightly parenthetical. The purely parenthetical word or expression is wholly detached from the essential meaning of the language in which it is found, and properly takes marks of parenthesis as evidence of this fact; the slightly parenthetical is in grammatical relation with some word or words in the language in which it appears.
An excellent illustration of such erroneous definitions of a parenthesis as is noted above, and of a lack of differentiation between the semicolon and the period, is found in the following sentence, which is the opening sentence of the preface to a work on the proper use of words, written, we believe, by one who was formerly a college professor of English:
69. The author’s main purpose in this book is to teach precision in writing; and of good writing (which, essentially, is clear thinking made visible) precision is the point of capital concern.
From our point of view the principal faults of this sentence are the following:
1. The and relation between the two clauses “and” here connects, is a strained one. The real relation between them would be better expressed by a period without the conjunction.
2. The explanatory group of words set off by parentheses is not a parenthesis, and therefore should not be enclosed in marks of parenthesis. Commas are the proper marks.