1: Ockham's razor (1287 – 1347): If you have two different, equally good explanations for an observation, investigate the simpler explanation first.
2: Einstein's razor (1879 - 1955): Everything should be made as simple as possible, but not simpler.
3: Sagan standard (1980): "Extraordinary claims require extraordinary evidence." It is not up to you to prove an extraordinary claim false. It is up to the person making an extraordinary claim to provide evidence.
4: Hitchen's razor (2007): "What can be asserted without evidence can also be dismissed without evidence."