I think it's because E contradicts the first premise, which isn't what we're supposed to be doing. We're trying to attack the reasoning of the argument (hence the gaps/assumptions) rather than the truth of the premises. We have to be charitable and assume the premises are true, and try to find out why the conclusion might be wrong even if they are true. idk if that makes sense my brain is fried too lol

i'm studying for june and probably august in pgh