Context: Character A has just told the character who is the narrator that he has the talent to be an assassin. Being a normal person, and thus taken off guard, the narrator responds with the following:
From what I can tell, the narrator says something along the lines of:
"Haha……, it’s nearly impossible for me to understand the thing you’re not saying that I have. In the first place, on what basis do you say that……"
I think that the italicized part of the English sentence corresponds to the bold bit in the Japanese which emphasizes the narrator's disbelief, but I'm unsure if my understanding of the Japanese is correct in terms of the sentence being an emphasized negative sentence, a double negative, or a positive created from two negatives.
What is the correct way of understanding the above Japanese sentence, given the context?