Can't Prove a Negative...
This mistake is known as 'proving a negative'. It is by definition impossible to prove something does not exist. It is always possible there is some place you haven't looked. I can't prove dragons and fairies don't exist somehow, somewhere (perhaps on some other planet, or in a parallel universe). But that's no reason to take seriously the claim that they do.
Sorry, folks, it doesn't work that way. Carl Sagan said it best: 'extraordinary claims require extraordinary evidence'. You make the positive claim, you provide the evidence. Nobody has any requirement to prove you are wrong; you have to prove you are right.
If you claim dragons and fairies exist, or that the mossad is behind 9/11, or that the income tax is a gigantic fraud by tens of thousands of people, it's not for me to prove these things are false and that no fairies, mossad conspiracy, or massive government fraud that has been going on for 100+ years. It's for you to prove these thing do exist. There is no point taking your claim seriously, otherwise.
The reason conspiracy theorists, from 9/11 "truthers" to tax deniers, constantly demand that someone prove to them 9/11 was not a conspiracy, or that the income tax is real, instread of defending their position by providing evidence it is correct, is simple: they can't do the latter, and thus presume that 'attack is the best defense'.
Labels: argument from ignorance