As we learn more and more about the solar system, the reality-check that our theories have to pass becomes more and more stringent. This is one reason why scientists have a habit of opening up old questions that everybody assumed were settled long ago, and deciding that they weren’t. It doesn’t mean the scientists are incompetent: it demonstrates their willingness to contemplate new evidence and re-examine old conclusion in its light. Science certainly does not claim to get things right, but it has a good record of ruling out ways to get things wrong.

-- The Science of Discworld, Terry Pratchett, Ian Stewart and Jack Cohen