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President-elect Donald Trump spent some of his weekend advancing the baseless conspiracy theory that several million illegal voters supported his opponent, accounting for her win in the national vote. Monday night, apparently angered by media coverage of his bizarre claims, he returned to the subject through a series of retweets.

It is true that journalists and election experts have examined Trump’s claims and found them to be baseless. Trump’s spokesman has offered up “evidence” for massive voter fraud, but the supposed evidence turns out to be a simple misreading of the facts. Trump’s response to this rebuttal is to demand proof to the contrary:

"@HighonHillcrest: @jeffzeleny what PROOF do u have DonaldTrump did not suffer from millions of FRAUD votes? Journalist? Do your job! @CNN" — Donald J. Trump (@realDonaldTrump) November 29, 2016

"@JoeBowman12: @jeffzeleny just another generic CNN part time wannabe journalist !" @CNN still doesn't get it. They will never learn! — Donald J. Trump (@realDonaldTrump) November 29, 2016

"@FiIibuster: @jeffzeleny Pathetic - you have no sufficient evidence that Donald Trump did not suffer from voter fraud, shame! Bad reporter. — Donald J. Trump (@realDonaldTrump) November 29, 2016

It is, of course, generally impossible to prove a negative. This is why Ted Cruz was unable to prove that his father did not help plot the assassination of John F. Kennedy, another favorite Trump conspiracy. Nor has China ever supplied proof that it did not create the theory of anthropomorphic global warming in order to gain an economic advantage. It is not a reliable method to assess the validity of a claim. Unfortunately, Trump has not yet hired a formal logician.