A student raised a tough question while I was teaching formal fallacies: couldn't the statement that "some cats are not tabbies" be made with confidence upon seeing a single cat that is not a tabby? Why would anything need to be known about the entire class of tabbies?
My first instinct was to say that his line of thought obverts the original proposition, making it essentially "some cats are non-tabby cats," which doesn't have a distributed predicate. How should he think of it instead, then? I've always had problems understanding or explaining O propositions at an intuitive level. Does it come down to existential import? Is it accurate to say, e.g., that you couldn't claim that the Holy Grail is not on my bookshelf unless you thoroughly searched it because it is unknown whether one or zero Holy Grails exist?
Does anyone have a better explanation for this?