Transcript: There is no science without evidence. When a scientist makes an assertion he must back it up with evidence. The evidence could be evidence that is physical evidence. It could be evidence gathered from telescopes, or microscopes, or other mechanisms we have to extend our senses. But a scientist must always back up what they say with real data. For instance, 200 years ago Jean Batiste Biot in France saw stones that fell from the sky. Nobody thought that stones could fall from the sky. But by gathering the eye witness accounts of many villagers and by gathering up fragments of stones that had no known terrestrial composition, he was able to prove that meteorites do exist, and they do indeed fall from the sky. Contrast this with the idea of UFOs, unidentified flying objects. Eye witness reports of UFOs have been piling up for decades, yet there has never been a single confirmed case where physical evidence has been evaluated independently by scientists leading to support for the idea of alien visitations. And so on through the whole edifice of science we can not understand anything without relying on evidence.