After apologizing for his ignorance, and reminding the audience that slavery was a poor school for the human intellect and heart, he proceeded to narrate some of the facts in his own history as a slave, and in the course of his speech gave utterance to many noble thoughts and thrilling reflections. (Preface.4)
Partly, Douglass is just being humble. He's giving a speech in front of a big audience, and he's never had much practice or training for that kind of thing. So he wants to remind his listeners not to judge him too harshly. At the same time, though, when Douglass calls slavery a "poor school for the human intellect and heart," he's reminding people that while slaves might often not seem to be as smart or as well-spoken as white people, this isn't their fault. Instead, it's the fault of the masters who enslaved them. After all, while Southerners would often claim that black people should be slaves because they were born inferior, Douglass thinks this is backwards: slaves aren't born inferior, but rather it's slavery that makes them inferior.
I look upon my departure from Colonel Lloyd's plantation as one of the most interesting events of my life. It is possible, and even quite probable, that but for the mere circumstance of being removed from that plantation to Baltimore, I should have to-day, instead of being here seated by my own table, in the enjoyment of freedom and the happiness of home, writing this Narrative, been confined in the galling chains of slavery. (5.11)
There are many reasons Douglass might still be a slave if he had never left Colonel Lloyd's plantation, but his comparison of the chains and the writing table shows one of the most important: it was only after he went to Baltimore that it became possible for him to expand his horizons and learn to read and write.
Captain Thomas Auld
"Learning would spoil the best n***** in the world. Now," said he, "if you teach that n***** (speaking of myself) how to read, there would be no keeping him. It would forever unfit him to be a slave. He would at once become unmanageable, and of no value to his master. As to himself, it could do him no good, but a great deal of harm. It would make him discontented and unhappy." These words sank deep into my heart, stirred up sentiments within that lay slumbering, and called into existence an entirely new train of thought. It was a new and special revelation, explaining dark and mysterious things, with which my youthful understanding had struggled, but struggled in vain. I now understood what had been to me a most perplexing difficulty--to wit, the white man's power to enslave the black man. (6.3)
Mr. Auld accidentally teaches Douglass why it's so important that slaves be kept illiterate. If a slave learned to read, he would no longer be satisfied to be a slave. In Mr. Auld's mind, of course, this would "ruin" him. Douglass learns an important lesson here about how the slave-masters keep their slaves from rebelling and running away. White men only have the power to enslave black people if they can keep them from getting educated.
Whilst I was saddened by the thought of losing the aid of my kind mistress, I was gladdened by the invaluable instruction which, by the merest accident, I had gained from my master. Though conscious of the difficulty of learning without a teacher, I set out with high hope, and a fixed purpose, at whatever cost of trouble, to learn how to read. The very decided manner with which he spoke, and strove to impress his wife with the evil consequences of giving me instruction, served to convince me that he was deeply sensible of the truths he was uttering. It gave me the best assurance that I might rely with the utmost confidence on the results which, he said, would flow from teaching me to read. (6.3)
When Mr. Auld prevents Mrs. Auld from teaching Douglass how to read, he's disappointed that his education has been interrupted. But even though it becomes more difficult to learn to read after that, Douglass learns something even more important from the experience: the power of education. The fact that Mr. Auld doesn't want him to read shows him that there's something valuable there, and makes him want to learn to read even more.
The plan which I adopted, and the one by which I was most successful, was that of making friends of all the little white boys whom I met in the street. As many of these as I could, I converted into teachers. With their kindly aid, obtained at different times and in different places, I finally succeeded in learning to read. (7.4)
When Mrs. Auld stops teaching Douglass to read, he has to find other ways to learn his ABCs, and he eventually does it by making friends with street kids. There's a certain irony in this. Mrs. Auld started teaching him to read before her husband "taught" her that the right way to treat slaves was as animals. So it turns out that his best teachers will be not grownups but children: white kids who haven't yet "learned" the lesson that Mrs. Auld has.
The first step in her downward course was in her ceasing to instruct me. She now commenced to practise her husband's precepts. She finally became even more violent in her opposition than her husband himself. She was not satisfied with simply doing as well as he had commanded; she seemed anxious to do better. Nothing seemed to make her angrier than to see me with a newspaper. She seemed to think that here lay the danger. I have had her rush at me with a face made all up of fury, and snatch from me a newspaper, in a manner that fully revealed her apprehension. She was an apt woman; and a little experience soon demonstrated, to her satisfaction, that education and slavery were incompatible with each other. (7.2)
When Mrs. Auld stops teaching Douglass, he learns a valuable lesson: education and slavery are incompatible. For Mrs. Auld, the more she learns about slavery, the more fearful and angry she becomes.
In moments of agony, I envied my fellow-slaves for their stupidity. I have often wished myself a beast. I preferred the condition of the meanest reptile to my own. Any thing, no matter what, to get rid of thinking! It was this everlasting thinking of my condition that tormented me. (7.6)
Douglass works hard for his education, but knowledge comes at a cost. While his learning makes him unwilling to be a slave (which ultimately leads him to his freedom), it also makes his life even harder to endure while he still is a slave. Sometimes he envies the slaves who are uneducated enough to be happy with their lot. The fact that Douglass wants to improve himself makes it all the more painful not to be able to do so.
We used frequently to talk about the fight with Covey, and as often as we did so, he would claim my success as the result of the roots which he gave me. This superstition is very common among the more ignorant slaves. A slave seldom dies but that his death is attributed to trickery. (10.22)
When Douglass achieves his victory over Covey, he attributes it to his vow to no longer be a slave. But the other slaves cannot accept this explanation; they decide that the root Sandy gave him must have been magic. In other words, one of the reasons the other slaves can't follow Douglass's example and stand up for their freedom is that they're too ignorant. Since they don't have the education he has, they superstitiously attribute his success to magic.
These dear souls came not to Sabbath school because it was popular to do so, nor did I teach them because it was reputable to be thus engaged. Every moment they spent in that school, they were liable to be taken up, and given thirty-nine lashes. They came because they wished to learn. Their minds had been starved by their cruel masters. They had been shut up in mental darkness. I taught them, because it was the delight of my soul to be doing something that looked like bettering the condition of my race. (10.23)
When Douglass starts a little school to teach the other slaves how to read, he notices that they don't come because they expect to get anything tangible out of it. In fact, they put themselves in great danger by coming to his school. But they still come, and Douglass reflects that this is because education is something that all human beings desire, even (or especially) when they've never had it.
In about four months after I went to New Bedford, there came a young man to me, and inquired if I did not wish to take the "Liberator." I told him I did; but, just having made my escape from slavery, I remarked that I was unable to pay for it then. I, however, finally became a subscriber to it. The paper came, and I read it from week to week with such feelings as it would be quite idle for me to attempt to describe. The paper became my meat and my drink. My soul was set all on fire. Its sympathy for my brethren in bonds--its scathing denunciations of slaveholders--its faithful exposures of slavery--and its powerful attacks upon the upholders of the institution--sent a thrill of joy through my soul, such as I had never felt before! I had not long been a reader of the "Liberator," before I got a pretty correct idea of the principles, measures and spirit of the anti-slavery reform. I took right hold of the cause. (11.17)
Reading an anti-slavery newspaper doesn't just teach Douglass that slavery is wrong; after all, he doesn't need anyone to tell him how messed up it is. Instead, reading the abolitionist Liberator gives him a cause, an ambition to do more than simply improve himself. He joins the abolitionist movement and starts working to abolish slavery as an institution.