If we change the form of No. 87, we may perhaps see somewhat more clearly why this grouping is more natural than would be a grouping made by a comma after the verb (came):
87-2. He did not come to teach, but to be taught.
No. 87-2 seems to show that the grouping made by the comma in No. 87 is a natural grouping.
In spite of Mr. Wilson’s rule, and of our reasoning to explain it, we believe that each mode of punctuating the following sentence is correct, each depending upon where the emphasis is to be laid:
88. The book’s primary aim is, not to convince the skeptic, but to solve the difficulties of the best-thinking men.
88-1. The book’s primary aim is not to convince the skeptic, but to solve the difficulties of the best-thinking men.
In No. 88 the emphasis is placed upon “to solve the difficulties,” the preceding group being thrown in for contrast, thus heightening the effect of the statement made in the next group of words.
In No. 88-1 the emphasis falls upon “not to convince the skeptic,” just as it would if written, “It is not the book’s aim to convince the skeptic.”
Let us note carefully that the mode of punctuation in each of the two preceding sentences would grow out of the context, which would clearly tell where the emphasis was to be laid.
We have dwelt upon this punctuation in order to emphasize a purpose of punctuation too often overlooked.