The statement that best explains why this is an example of a counterclaim by Douglass is:
Douglass addresses a potential argument of the other side and makes a case against it.
In this excerpt, Douglass anticipates the argument of someone who criticizes abolitionists for being too denouncing rather than persuasive. He counters this by asserting that the issue of slavery is so clear that there is no need for further argument about the humanity of the slaves, indicating that his position is strong and well-founded.