Answer:
It asserts that African people are no less human than white settlers.
It contends that white settlers should treat African people more respectfully.
Explanation:
In Samuel Sewall's pamphlet "The Selling of Joseph", the author argues his case against the discriminatory act of practicing slavery. This pamphlet was also the very first anti-slavery text in New England.
As seen in the given excerpt, Sewall began to fiercely debate that the "Ethiopians, as black as they are, [are] the Sons and Daughters of the First Adam" establishing the fact that they are also a descendant of the first man on earth. And if they are from Adam, they are also the same as the whites who are also the descendant of the same Adam that God created and put in the Garden of Eden. So, the skin color of a person does not matter and it definitely does not determine what life he is to lead or be given. He also has the right to live the same life the whites live.
So, Judge Samuel Sewall's assertion and his persuasion are that the African people are the same as the white settlers and that they have the right to be treated with respect too.