An retorsion argument consists in pointing out how a claim is self-defeating.

Of course, I accept that there are sound arguments of this type that don't misrepresent the original position. But they seem rare to me.

Especially in the context of epistemology I've encountered loads of fishy retorsion arguments.

Alice: *"Nothing is certain."*  

Bob: *"Then 'nothing is certain' is not certain, though you claim it to be! Refuted!"*

Put in a more general form :   

> 
> Alice claims that all truths must fulfill a certain condition.
> 
> 
> Bob counters by alleging that Alice makes a claim to a truth that does not fulfill this very condition. (S1)
> 
> 
> 

With this you can construct examples for "scientism," Hume's fork, relativism, etc.

Are retorsion arguments fitting the scheme S1 always unsound?

To me they feel *obviously* wrong (but it's just an intuition, vaguely in the direction that Alice doesn't really try to make a truth claim but just gives a warning "we shouldn't accept purported truths of this type"). If you share my view, can you help me to formulate a precise, rigorous refutation?