Such will try to discredit the shocking tales of slaveholding cruelty which are recorded in this truthful Narrative; but they will labor in vain. Mr. Douglass has frankly disclosed the place of his birth, the names of those who claimed ownership in his body and soul, and the names also of those who committed the crimes which he has alleged against them. His statements, therefore, may easily be disproved, if they are untrue. (Preface. 10)
William Lloyd Garrison knows that Douglass's enemies will try to claim that Douglass's narrative wasn't true. (And they did.) Pro-slavery debaters would try to deny or downplay the horrors of slavery, so Garrison understood that all Douglass really had to do was tell the truth. By encouraging Douglass's opponents to try to disprove him, he showed that he and Douglass had nothing to fear from the truth.
Letter from Wendell Phillips
You remember the old fable of 'The Man and the Lion,' where the lion complained that he should not be so misrepresented 'when the lions wrote history.' I am glad the time has come when the 'lions write history.' (Letter.1-2)
Phillips is reminding his audience of an old fable about a lion who complains that when humans write stories, they always make the lion into the villain. His point is simple: since history books are always written by the winners, the only people who had ever written books about slavery were white people – mostly white southerners who owned slaves. Douglass's book is significant because it's one of the most important examples of a black slave writing about his own experience of slavery.
After all, I shall read your book with trembling for you. Some years ago, when you were beginning to tell me your real name and birthplace, you may remember I stopped you, and preferred to remain ignorant of all. With the exception of a vague description, so I continued, till the other day, when you read me your memoirs. I hardly knew, at the time, whether to thank you or not for the sight of them, when I reflected that it was still dangerous, in Massachusetts, for honest men to tell their names! (Letter.7)
Wendell Phillips wants to remind us that Douglass's decision to tell the truth about his experience (and to publish under his own name) is a very courageous one. Even for freed slaves, telling the truth about their lives was dangerous precisely because it was so powerful. Not only would Douglass's enemies try to find him and kidnap him back into slavery, but identifying himself in print made it easier for them to do it.
Again, we have known you long, and can put the most entire confidence in your truth, candor, and sincerity. Every one who has heard you speak has felt, and, I am confident, every one who reads your book will feel, persuaded that you give them a fair specimen of the whole truth. (Letter.5)
Wendell Phillips makes the case that we should trust Douglass's story because of his character. If we know him to be a trustworthy person, he suggests, his book can be treated as true. But part of how we know that a person is trustworthy is our experience of the person himself: Douglass's audience can tell just from listening to him that he must be telling the truth.
This is the penalty of telling the truth, of telling the simple truth, in answer to a series of plain questions. (3.5)
Douglass is telling the story of a slave who made the mistake of telling his master (whom he didn't recognize) the truth about his poor treatment. The slave was severely punished, showing that slave owners aren't interested in the truth. Instead of admitting that slavery is an oppressive system, the slave masters require that their slaves flatter them. This is probably related to the ways slave owners treat religion, using their version of Christianity to make themselves feel better, rather than facing the truth about slavery.
The slaveholders have been known to send in spies among their slaves, to ascertain their views and feelings in regard to their condition. The frequency of this has had the effect to establish among the slaves the maxim, that a still tongue makes a wise head. They suppress the truth rather than take the consequences of telling it, and in so doing prove themselves a part of the human family. (3.6)
Because slaves can be punished for telling the truth when it isn't what their masters want to hear, they learn to keep quiet, lying if they have to. It's the slave master's inhumanity that forces the slave to lie; Douglass emphasizes that the slaves are only doing what is natural for normal people: staying out of trouble.
Mr. Gore acted fully up to the maxim laid down by slaveholders, -- "It is better that a dozen slaves should suffer under the lash, than that the overseer should be convicted, in the presence of the slaves, of having been at fault." No matter how innocent a slave might be -- it availed him nothing, when accused by Mr. Gore of any misdemeanor. To be accused was to be convicted, and to be convicted was to be punished; the one always following the other with immutable certainty. (4.2)
As Mr. Gore's conduct demonstrates, the rule for slaves is simple: the slave is always wrong and the master is always right. Douglass is showing us that slavery doesn't operate according to any principles of justice or fairness but is, instead, simply about power.
I may be deemed superstitious, and even egotistical, in regarding this event as a special interposition of divine Providence in my favor. But I should be false to the earliest sentiments of my soul, if I suppressed the opinion. I prefer to be true to myself, even at the hazard of incurring the ridicule of others, rather than to be false, and incur my own abhorrence. From my earliest recollection, I date the entertainment of a deep conviction that slavery would not always be able to hold me within its foul embrace; and in the darkest hours of my career in slavery, this living word of faith and spirit of hope departed not from me, but remained like ministering angels to cheer me through the gloom. This good spirit was from God, and to him I offer thanksgiving and praise." (5.12)
Douglass has a high opinion of himself, and he knows that if he tells us that he always believed it was his destiny to be free, we might think he's a little arrogant or even crazy. But he tells us anyway, which shows us something important about him: he believes in the saying "To thine own self be true." And maybe he's right!
Just about this time, I got hold of a book entitled "The Columbian Orator." Every opportunity I got, I used to read this book. Among much of other interesting matter, I found in it a dialogue between a master and his slave. The slave was represented as having run away from his master three times. The dialogue represented the conversation which took place between them, when the slave was retaken the third time. In this dialogue, the whole argument in behalf of slavery was brought forward by the master, all of which was disposed of by the slave. The slave was made to say some very smart as well as impressive things in reply to his master--things which had the desired though unexpected effect; for the conversation resulted in the voluntary emancipation of the slave on the part of the master. . . . The moral which I gained from the dialogue was the power of truth over the conscience of even a slaveholder. (7.5)
What Douglass learns from this book is that, when it's given a chance to be heard, the truth will always win out. When the slave and the slave owner argue over whether slavery is justified, the slave has already won: if he's allowed to speak, he can tell the truth about slavery, and no argument can refute that. This helps us understand why Douglass associates slavery so closely with deception: the only way slave owners can defend slavery is by lying about it.
The holidays are part and parcel of the gross fraud, wrong, and inhumanity of slavery. They are professedly a custom established by the benevolence of the slaveholders; but I undertake to say, it is the result of selfishness, and one of the grossest frauds committed upon the down-trodden slave. They do not give the slaves this time because they would not like to have their work during its continuance, but because they know it would be unsafe to deprive them of it. This will be seen by the fact, that the slaveholders like to have their slaves spend those days just in such a manner as to make them as glad of their ending as of their beginning. Their object seems to be, to disgust their slaves with freedom, by plunging them into the lowest depths of dissipation. (10.17)
Douglass wants to show that the slave masters don't maintain their iron grip on their slaves by force alone. An important way they keep power is by deceiving their slaves into not understanding what slavery is. When they give their slaves a taste of freedom at Christmas, they trick them into getting drunk and degrading themselves, so that afterwards (when they're hung over) they associate those bad feelings with freedom and decide they don't want to be free at all. Slavery, in other words, depends on deception.
I remained firm, and, according to my resolution, on the third day of September, 1838, I left my chains, and succeeded in reaching New York without the slightest interruption of any kind. How I did so,--what means I adopted,--what direction I travelled, and by what mode of conveyance,--I must leave unexplained, for the reasons before mentioned. (11.5)
When Douglass escapes, he is still bound to a certain kind of secrecy. If he tells the truth about what happened and how he escaped, it could get the friends who helped him into trouble.