I found this word while looking at this description of interesting words from Nabokov’s Speak, Memory. Nabokov is of course notorious for his capacious vocabulary, which he unsympathetically inflicts on the poor reader. It’s wonderful. Anyway, this word is not exactly a contronym, but I find its two definitions a pleasing apparent juxtaposition (though no juxtaposition at all for a philosopher!):
quiddity (noun): 1. The real nature of a thing; the essence; 2. a hairsplitting distinction; a quibble.