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Is Confusing Necessary for Sufficient the Contrapositive for Confusing Sufficient for Necessary?

LouislepauvreLouislepauvre Alum Member

Hey everybody!

I've been thinking about this for a while, and I just re-watched the video on Contrapositive Mistakes, which has cemented my feelings. Do you know in Flaw questions where the answers will say something like, "Takes a necessary condition for an argument's being inadequate to be a sufficient condition for an argument's being inadequate"? Sometimes answer choices in Flaw questions really trip me up, and one thing that always confused me is: what's the difference between confusing sufficient for necessary as opposed to confusing necessary for sufficient? I read a forum on PowerScore about this that claimed that they are not the same, but to me, they are, because they are the contrapositive of each other.

Confusing Sufficient for Necessary


This is making the sufficient necessary because after you fail A you fail B, instead of the rule falling away as it's supposed to.

Confusing Necessary for Sufficient


Here it's taking the necessary condition and moving it to the sufficient, which is wrong.

But, isn't A/---->/B just the contrapositive of B---->A? And therefore, in Flaw questions that use this incorrect form of logical reasoning, wouldn't either answer choice (confusing necessary for sufficient OR sufficient for necessary) be correct? Has anybody else ever thought about this?


  • sk144404sk144404 Core Member
    237 karma

    To get the contrapositive you flip the conditions positions then negate.

  • equallyyokedequallyyoked Alum Member
    374 karma

    You cannot look at the necessary and assume that the sufficient happened. That would be going backwards in logic which makes an answer choice like that wrong.

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