Skarin said:
But you can prove that no fraction exists that's equal to the square root of two.
Personally, I think that things which are removed from reality aren't significant to prove.
The thing about numbers is we basically shave off all the details to make numbers a useful tool. Two apples = 2. That the apples are not truly identical in real life on the grounds that their molecular structure is not exactly the same, they have a different history, what they might have been before they were apples (1 tree bearing nutrients that would later become apples), is removed so we have a nice, workable 2.
That established, we determine that no fraction exists that equal to the square of two, but who cares? Numbers are a tool we use to measure things, not real in and of themselves. The properties they exhibit seem very real, but given that we have established a very crude and firm set of limitations by which they must operate, how can they not? It's reality within the parameters of the unreal.
So, along these lines, I'm not getting as far as "can you prove a universal negative." I'm stopping at "can you prove universal" (anything). Given the limits of human perception, what's really provable, down to its finest detail?
Nothing.
That said, we can achieve a remarkable amount with our crude abstractions. That of all the insanity that the human mind generates in its relentless push to reduce reality into something workable, the true miracle is that some of this insanity seems to work, even capable of producing remarkable results. When things seem to work, this is the essence of "proof."
However, for scientist and philosopher alike, all proof exists to disprove should new details come to light. The moment you believe you have found a universal, irregardless of our limited means of perceiving the universe, therein lay a probable instance of pure delusion.