Wow, you're not joking. That actually was terrible. He does sort of have a point that if you positively prove something that excludes something else, you have essentially proven a negative. It doesn't work for stuff outside of abstract logical rules though and the way he argued his case is pretty bad.
you can prove that you aren't nonexistent. Congratulations, you've just proven a negative. The beautiful part is that you can do this trick with absolutely any proposition whatsoever. Prove P is true and you can prove that P is not false. Some people seem to think that you can't prove a specific sort of negative claim, namely that a thing does not exist. So it is impossible to prove that Santa Claus, unicorns, the Loch Ness Monster, God, pink elephants, WMD in Iraq, and Bigfoot don't exist. Of course, this rather depends on what one has in mind by 'prove.'
"Can you construct a valid deductive argument with all true premises that yields the conclusion that there are no unicorns? Sure. Here's one, using the valid inference procedure of modus tollens: 1. If unicorns had existed, then there is evidence in the fossil record. 2. There is no evidence of unicorns in the fossil record. 3. Therefore, unicorns never existed."
I bet if we wait 10 years, we'll find evidence of a new creature in the the fossil record. Prior to that point, we could "prove" that the creature doesn't exist?
"Someone might object that that was a bit too fast—after all, I didn't prove that the two premises were true. I just asserted that they were true. Well, that's right. However, it would be a grievous mistake to insist that someone prove all the premises of any argument they might give."
Hahaha. In other words, some might object that to prove something we need to prove something. How about we just don't prove it and say we did?
They'll fine you, and if you don't pay they'll put a lien on your house. You can't really just ignore them.