Full Idea
Einstein assumed that when each observer measures the speed of light in a vacuum, they find the same value, regardless of their speed; and that measurements will lead to agreement on the laws of physics.
Gist of Idea
Assume the speed of light is constant for all observers, and the laws of physics are the same
Source
report of Albert Einstein (works [1915]) by Graham Farmelo - The Strangest Man 03
Book Reference
Farmelo,Graham: 'The Strangest Man [Paul Dirac]' [Faber 2009], p.40
A Reaction
So are the laws of physics constant for all observers, irrespective of their speed?