A synonym for mastery might be:

Two that immediately come to mind are virtuosity and dexterity.

A synonym for mastery might be proficiency or comprehension.

Explanation:

Mastery means to have "comprehensive knowledge or skill in a subject or accomplishment. " (Source: Oxford).

Proficiency means to have "a high degree of competence or skill; expertise. " (Source: Oxford).

Comprehension means to have "the action or capability of understanding something. ".

Comprehension may be more mental focused than the connotation of mastery, but it still fits.

        its hard for some people to get along and that’s the case for douglass and canot. most of the time when people do not agree with each other its because they have different presperctives, and that is the case here.

captain canot was never a slave so but he was a slaver and a slave ship captain and just like the slave master's he tought it was it was totally normal, he used captain conot is a cruel man who enjoys whipping slaves but never   got whipped he saw it has a destiny for the slave to be a slave and the slaver to be a slaver   but douglass has suffered whoppings cut   even for no reason sometimes. douglass believed that slavery was not natural to black people, they need to study and learn how to read and write. he knew they have rights.

it was really not the captain’s fault he was just brainwashed by the system and when your brainwashed by the system its hard not to be brainwashed. but the reason douglass wants this is because he has suffered he knows how it feels and will do anything to stop that.