Rhetoric lesson for Keith:
"negative proposition" itself is a solecism. just how does one propose a negative? how does one put forward an absence? oh sure, you can express a sentence like "it is not raining outside" but that in no way communicates logically or empirically what is, in fact, happening outside. it might be typhooning instead!
A proposition proposes, puts forward, offers, describes some specifiable state of affairs. to say "Zorg is not a woman" tells me nothing about who (or what) "Zorg" might be. "Zorg is a dog" is not logically equivalent to "Zorg is not a woman."