 Einstein thought that the key to achieving this aim lay in further expanding the range of reference frames in which the laws of physics take their canonical form, to include frames adapted to any arbitrary motions. More specifically, since the class of all continuous and differentiable coordinate systems includes as a proper subclass the coordinate systems adapted to any such frame of reference, if he could achieve a theory of gravitation, electromagnetism and mechanics that was generally covariant – its equations taking the same form in any coordinate system from this general class – then the complete relativity of motion would be achieved. If there are no special frames of reference in which the laws take on a simpler canonical form, there is no physical reason to consider any particular state or states of motion as privileged, nor deviations from those as representing ‘absolute motion’. (Here we are just laying out Einstein’s train of thought; later we will see reasons to question the last step.) And in 1915, Einstein achieved his aim in the General Theory of Relativity (GTR).