The emergence of an old problem: if there’s a problem with a reductio, what do you call it?

It's always fun to meet an old friend, even when "friend" means "difficulty". What do I call my problem with a reductio ad absurdum given that such "arguments" by definition are deductively invalid and thus not sound? A familiar problem in a new context.

Now on to negative assertions and Plato

My interpretation of Plato’s doctrine of negative assertions (or “not-being) in the Sophist at (roughly) 257e   Plato’s doctrine of negative assertions:   Stranger: May we not say that the existence of the not-beautiful is constituted by its being marked off from a single definite kind among existing things and again set in contrast with …