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I'm looking for a term that describes the logical argument I am making. Both to just have a way to describe it and ensure that it is indeed a sound argument.

Somewhat formally the argument is as follows. A person argues that A implies B. Now I show that B can be equally expressed as C (so now A must imply C as well). I ascertain that C should never be true. Therefore either A implies B is invalid or A is false.

As a situation, somebody is arguing in favour of a given requirement (of some specification or legal document). I ignore their reasoning and look solely at the implications of the requirement. I show that I can represent the requirement in a different, but equivalent fashion. The alternative representation is however very unexpected, somewhat ridiculous. Therefore I argue that you have to be either against the original requirement or in favour of this ridiculous requirement.

I've considered "reductio ad absurdum" but it feels wrong here. There could be a contradiction, but instead we could simply draw a ridiculous conclusion (in which case the argument is logically sound, but has no supporters).

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I've considered "reductio ad absurdum" but it feels wrong here.

Actually, it's exactly right here.

There could be a contradiction, but instead we could simply draw a ridiculous conclusion (in which case the argument is logically sound, but has no supporters).

No: the argument would be valid, but not sound. And that's the distinction that makes a difference. A reductio does not need to result in a contradiction; it can also result in an absurd conclusion which is clearly false. (This is technically a reductio ad falsam, but it is generally considered a subtype of reductio ad absurdam.

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    Okay, if I understand correctly, my opponent claims that A implies B is sound. Since I show B == C this means A implies C must also be sound. C is clearly false, thus A implies B cannot be sound. – edA-qa mort-ora-y Jan 22 '12 at 13:14
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    You got it! Classic reductio ad falsam. – Michael Dorfman Jan 22 '12 at 13:44

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