I read “ought to be able to” not as “you’re not allowed to reject the conclusion without rejecting a premise” so much as “you ought to be able to, so when you find you’re not able to, it should bother you; you have learned that there’s a key failing in your understanding of that area.”
I read “ought to be able to” not as “you’re not allowed to reject the conclusion without rejecting a premise” so much as “you ought to be able to, so when you find you’re not able to, it should bother you; you have learned that there’s a key failing in your understanding of that area.”