He professed both to abominate and despise all mystery, refinement, and intrigue, either in a prince or a minister. He could not tell what I meant by secrets of state, where an enemy, or some rival nation, were not in the case. He confined the knowledge of governing within very narrow bounds, to common sense and reason, to justice and lenity, to the speedy determination of civil and criminal causes. (2.7.5)
The King of Brobdingnag is the first, lesser example of the kind of moralist we see at his best in the Master Horse. He is aware that there is such a thing as falsehood, but he totally despises it. Oddly, there are laws on the books in Lilliput that seem to express similar sentiments. Gulliver remarks that the Lilliputians punish fraud with death. Yet, their Emperor and nobility lie all the time. Perhaps Swift is using this note about Lilliputian law to poke fun at the degeneration of England from its high principles to its current predicaments.
Part 3, Chapter 6
It is first agreed and settled among them, what suspected persons shall be accused of a plot; then, effectual care is taken to secure all their letters and papers, and put the owners in chains. These papers are delivered to a set of artists, very dexterous in finding out the mysterious meanings of words, syllables, and letters: for instance, they can discover a close stool, to signify a privy council; a flock of geese, a senate; a lame dog, an invader. (3.6.12)
Here, Gulliver is describing the process by which men accused of crimes against the state are often framed by their enemies. These enemies seize their papers and twist their words into a kind of fake code indicating wrongdoing. This kind of judicial lying truly disturbs Gulliver, because (obviously) the law is supposed to protect people. In this political climate, however, it's being used to persecute them instead.
But as to honour, justice, wisdom, and learning, they should not be taxed at all; because they are qualifications of so singular a kind, that no man will either allow them in his neighbour or value them in himself. (3.6.7)
The political projectors in Balnibarbi spend some time trying to figure out how to raise money. They immediately dismiss the idea of taxing good qualities in people – honor, justice, wisdom, and learning – because this kind of honesty is: (a) too rare, (b) would never be admitted by the guy's jealous neighbors.
Part 4, Chapter 3
The Master Horse
[The Master Horse] replied, "that I must needs be mistaken, or that I said the thing which was not;" for they have no word in their language to express lying or falsehood. "He knew it was impossible that there could be a country beyond the sea, or that a parcel of brutes could move a wooden vessel whither they pleased upon water. He was sure no Houyhnhnm alive could make such a vessel, nor would trust Yahoos to manage it." (4.3.4)
Master Horse is so shocked by Gulliver's stories of countries beyond the ocean that he actually imagines that Gulliver is lying – something that the Houyhnhnms could not previously think of. You say that the Master Horse has bit of cognitive dissonance, here, meaning he's hearing something that totally doesn't fit into his worldview. It's interesting to watch this happen to someone other than Gulliver for once.
Part 4, Chapter 4
The Master Horse
For [the Master Horse] argued thus: "that the use of speech was to make us understand one another, and to receive information of facts; now, if any one said the thing which was not, these ends were defeated, because I cannot properly be said to understand him; and I am so far from receiving information, that he leaves me worse than in ignorance; for I am led to believe a thing black, when it is white, and short, when it is long." And these were all the notions he had concerning that faculty of lying, so perfectly well understood, and so universally practised, among human creatures. (4.4.1)
The reason the Master Horse thinks that lying is impossible or against reason is because the point of speech is to communicate what you think. Why else would you talk? So, the thought of talking to obscure what you think seems totally inconceivable to the Master Horse.
It put me to the pains of many circumlocutions, to give my master a right idea of what I spoke; for their language does not abound in variety of words, because their wants and passions are fewer than among us. But it is impossible to express his noble resentment at our savage treatment of the Houyhnhnm race; particularly after I had explained the manner and use of castrating horses among us, to hinder them from propagating their kind, and to render them more servile. (4.4.4)
The Master Horse finds the idea of the European Yahoo's treatment of horses to be totally disgusting. The Horse is definitely not cool with the idea of breaking his European cousins to the saddle and castrating male horses to make them more peaceful. But at the end of the book, the Master Horse suggests using the same methods on Houyhnhnm Land Yahoos to make them more obedient. This seems pretty hypocritical to us – can it be that the Master Horse is learning deceit from Gulliver?
Part 4, Chapter 5
I said, "there was a society of men among us, bred up from their youth in the art of proving, by words multiplied for the purpose, that white is black, and black is white, according as they are paid. To this society all the rest of the people are slaves. (4.5.11)
This "society of men" is lawyers. Gulliver thinks that all lawyers must be deceitful because they're paid to argue, and it has nothing to do with personal conviction. And the Master Horse agrees that it is a perversion of law to make it subject to things like money and private interest.
Part 4, Chapter 7
But I must freely confess, that the many virtues of those excellent quadrupeds, placed in opposite view to human corruptions, had so far opened my eyes and enlarged my understanding, that I began to view the actions and passions of man in a very different light, and to think the honour of my own kind not worth managing; which, besides, it was impossible for me to do, before a person of so acute a judgment as my master, who daily convinced me of a thousand faults in myself, whereof I had not the least perception before, and which, with us, would never be numbered even among human infirmities. I had likewise learned, from his example, an utter detestation of all falsehood or disguise; and truth appeared so amiable to me, that I determined upon sacrificing every thing to it. (4.7.1)
Here, Gulliver is explaining why his tune has changed so much. When he lived in Brobdingnag, he tried to dress up England's customs to make them seem better to the critical Brobdingnagian King. But in Houyhnhnm Land, he doesn't bother. He just speaks directly about the many awful things he has seen in England. And yet, is it any more truthful only to reveal the bad things about a place and none of the good things? Gulliver seems to be falling into his own kind of irrational prejudice here.
Part 4, Chapter 8
I remember it was with extreme difficulty that I could bring my master to understand the meaning of the word opinion, or how a point could be disputable; because reason taught us to affirm or deny only where we are certain; and beyond our knowledge we cannot do either. So that controversies, wranglings, disputes, and positiveness, in false or dubious propositions, are evils unknown among the Houyhnhnms. (4.8.9)
The Houyhnhnms have no opinions, which Gulliver likes. But Swift was a quarrelsome old cuss; why else would he write such biting satires? Surely this criticism of opinion is at least a bit sarcastic?
Part 4, Chapter 12
Thus, gentle reader, I have given thee a faithful history of my travels for sixteen years and above seven months: wherein I have not been so studious of ornament as of truth. I could, perhaps, like others, have astonished thee with strange improbable tales; but I rather chose to relate plain matter of fact, in the simplest manner and style; because my principal design was to inform, and not to amuse thee. (4.12.1)
Inform us of what? What might Gulliver think he has taught us over the course of his travels? Do we get a different sense of Gulliver's character development as readers than he might think as the person who experienced all of these travels? And why does Gulliver take such pains to assure us that his work is true when this novel is fiction? Is Swift satirizing the style of traveler's narratives, or is he really making a claim for some higher truth in Gulliver's Travels? Or all of the above?
My reconcilement to the Yahoo kind in general might not be so difficult, if they would be content with those vices and follies only which nature has entitled them to. (4.12.13)
What Gulliver hates about the European Yahoos is that they have all the bad qualities of the Houyhnhnm Land Yahoos, but they still feel so much pride in themselves. This is the ultimate self-deception. Gulliver wants all Yahoos to admit how disgusting they/we are.