Slaves would often say that they were content and their masters were kind. Why? What is the slave maximum Douglass uses to explain this?

From the text:

It is partly in consequence of such facts, that slaves, when inquired of as to their condition and the character of their masters, almost universally say they are contented, and that their masters are kind. 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.