Bearing in mind that I have just walked a dozen miles and am sitting in Wetherspoons with the best part of a pint of Bath porter inside me pecking this out on my PDA:

There's a Latin or Greek word for suppressed premises which I can't remember, because I am not Gottlob Frege, But your suppressed premises (or those of your hypothetical arguing idiot) are that your knowledge etc is incorrigible and therefore X is certainly true. But how much of our knowledge is actually incorrigible?

I may not mean incorrigible. Time to haul myself up the hill I think.