@neologist,
The point I was
hammering home here was that we would consider a structurally analogous statement like "health could actually be another form of illness" to be totally ridiculous.
The confusion here by "naive realists" like Frank, is to assume that
concepts like "reality" or "existence" can be defined in
absolutist terms rather than relative ones like "health" and "illness".
(Wannabe intellectuals are invited refer to Quine's "Ontological Relativity" for the background to this. Quine was a leading logician of the 20th century).