This letter is dated 1727, written from Gulliver to Sympson. Gulliver is furious with Sympson’s edits of his book, protesting Sympson’s adjustments to his story, especially the addition of a passage praising the English Queen (though Gulliver says he respects the Queen, he insists he never would have praised her to the Houyhnhnms). He complains, too, that Sympson has muddled the details of his sea travel. He calls the book libelous. He has received a great deal of abuse for the book and everyone doubts the veracity of the account.
This letter introduces the theme of perspective. Though Sympson has just expressed his edition of the text, Gulliver is furious with his edits. By accusing Sympson of falsification and libel, this letter not only calls the truth of Sympson’s letter into question, it also implies that the text to come (as edited by Sympson) is itself somehow untrue, while also therefore implying that at least some part of the narrative is true—because why would Gulliver be angry about Sympson's "falsifications" if Gulliver's own story wasn't true?
Throughout the letter, Gulliver refers to human beings as Yahoos and laments the perverse world in which degenerate Houyhnhnms are enslaved by Yahoos. Though Gulliver acknowledges that he, too, is a Yahoo, he notes that he was elevated by his education among the Houyhnhnms, though some of that refinement has begun to erode during his time spent back among “your species…particularly those of my own family.”
At the same time, Gulliver’s crazy names for things and his insistent distancing of himself from human beings (“your species”) suggest that he may not be fully sane as he writes the letter. Is his claim to truth undercut by a potentially insane perspective?