The principle of relativity is derived from Galileo's ideas. It is a frame of reference for describing scientific enquiries themselves which allow the laws of physics to be written and apply everywhere. For example, Newtonian mechanics are a form of the principle of relativity in action. When the work of Einstein expanded our knowledge of physics in the twentieth century, his principles had to conform with Newtonian ideas somehow so that the same frame of reference, or principle, was maintained.