That's interesting. I would be inclined to say that necessarily nothing can both be a human hand an a parrot, so even if the skeptic is right, we can still know that the hand in question is not a parrot. Rather than skepticism, I think the worry would be how we can know that the concept of human hand is incompatible with the concept of parrot. I agree with Allen, however, that in order to answer this question we'd have to get clearer on the notions of "objective" and "prove" etc. But parrots are a kind of bird (the quickest web definition states: " usually brightly colored zygodactyl tropical birds with short hooked beaks and the ability to mimic sounds") and human hands aren't birds at all. So either you're wrong that you're talking about a human hand, or you're wrong that it is a parrot. And even a skeptic could grant this....

Hi Kobe: There are a lot of interesting issues packed into your question. Let me try to unpack a couple of them. Philosophers tend to think that knowledge just is a special kind of belief, e.g., a belief that is true and that one is justified in holding. So for most philosophers to conclude that knowledge is nothing more than a belief isn't that worrisome. What is more worrisome is whether knowledge is nothing more than "mere opinion" or "unjustified belief". This may be what you have in mind. But then the question is why you think that the argument you give leads to the conclusion that knowledge is nothing more than mere opinion. To begin, most philosophers think that we have lots of knowledge that is not based on axioms. For example, when I believe that there is a pencil on my desk based on my perception of the pencil, this belief counts as knowledge even though it wasn't derived from an axiom. The domain where it is more plausible that knowledge is axiomatic is in logic and math ...