Sentence:  In each case, we are reborn -- a fact that explains Thoreau's preference here for the <WSD>father tongue</WSD> over the mother tongue, as Cavell explains: "A son of man is born of woman; but rebirth, according to our Bible, is the business of the father." The rebirth of Walden will involve baptism in the waters of Walden Pond, and, by extension, in Walden, the book Thoreau endeavors to write in the <WSD>father tongue</WSD>. 
Meaning of the word marked with <WSD> tags:  The form of language that is acquired through education and books, as opposed to the dialect one grows up speaking; formal or educated language.