 In 1907 Einstein published his first gravitation theory (Einstein 1907), treating the gravitational field as a scalar field that also represented the (now variable and frame-dependent) speed of light. Einstein viewed the theory as only a first step on the road to eliminating absolute motion. In the 1907 theory, the theory’s equations take the same form in any inertial or uniformly accelerating frame of reference. One might say that this theory reduces the class of absolute motions, leaving only rotation and other non-uniform accelerations as absolute. But, Einstein reasoned, if uniform acceleration can be regarded as equivalent to being at rest in a constant gravitational field, why should it not be possible also to regard inertial effects from these other, non-uniform motions as similarly equivalent to “being at rest in a (variable) gravitational field”? Thus Einstein set himself the goal of expanding the principle of equivalence to embrace all forms of ‘accelerated’ motion.