Actually they have a section in their page that deals with this point. There is some validity to the "cannot prove a negative" claim, but I think they are right here. WRT your example: given that the event is quantified/qualified, you can indeed prove (potentially) that it did not happen. It all depends on how the positive or negative are phrased. The "cannot prove a negative" issue is I think better seen as a part of the problem of induction -- say you make the claim that all ravens are black. I respond: prove there are no black ravens. You cannot prove through induction that this is true. However, it is worthy of note that you might have other options, such as the use of biological/genetic theories of physical traits and their source, combined with a definition of raven-ness, to demonstrate that indeed all ravens are black, and that if a creature is found that is not black, then it is not a raven. This is a theoretical adventure of course, since I do not know anything about whether albino traits are possible among ravens.
--ravi
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