Quote 46
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.
Quote 47
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.