Part of the problem for platforms like YouTube and Facebook — which has also pledged to clean up misinformation that could lead to real-world harm — is that the definition of “harmful” misinformation is circular. There is no inherent reason that a video questioning the official 9/11 narrative is more dangerous than a video asserting the existence of U.F.O.s or Bigfoot. A conspiracy theory is harmful if it results in harm — at which point it’s often too late for platforms to act. – The New York Times