I'll see your point but raise you: the absolute soundness of an argument is distinct from our ability to determine that soundness.

This isn't just a technical incompleteness argument; the law is not written using a propositional calculus, but rather an imprecise semantic encoding (which we might refer to as "Legal English") that is subject to multiple interpretations.

So in the case of law it is possible/likely/inevitable that the argument is, by the use of natural language, itself ambiguous, so the soundness thereof is a vector state.

Or to put it in simpler terms: it is all a matter of opinion.