zlacker

  person 1: Aliens have visited our city.
  person 2: Is there proof of that? 
            Probably just an optical illusion or something.
  person 1: Can you prove aliens didn't visit? 
            What is your math for optical illusion?

I'm sorry, but the burden of proof lies with the person making a claim, not with detractors.

>>DenisM+(OP)
Yes, and applecore made a claim that David Chapman's observed experience of having his brain slow down was caused by the placebo effect. I asked for proof of that.

Also, when people assert that UFOs are probably just an optical illusion, they are (or at least should be) doing that based on the long observational baseline of many investigated incidents which indeed turn out to be optical illusions. That is to say, it's legitimate to say that perceived UFOs are probably optical illusions it's because there's real data demonstrating that perceived UFOs really are optical illusions.

The illegitimate version of that would be saying it based on general arrogance, which is frequently a barrier to scientific progress. See, e.g., the story of Nobel Prize winner Barry Marshall.