HELP ME PLEASE! :O In this excerpt from paragraph 2, what does Douglass mean when he describes his mistress as an apt woman?

My mistress was, as I have said, a kind and tender-hearted woman; and in the simplicity of her soul she commenced, when I first went to live with her, to treat me as she supposed one human being ought to treat another. In entering upon the duties of a slaveholder, she did not seem to perceive that I sustained to her the relation of a mere chattel and that for her to treat me as a human being was not only wrong but dangerously so. Slavery proved as injurious to her as it did to me.

mischievous
deceptive
intelligent
admirable -This is wrong X-