I wonder what really motivated Einstein to formulate his general theory of relativity: a) would be due to the principle of equivalence? b) it would be because after the relativistic formulation of Maxwell's equations, there was only the classical gravitation with the factor 1 / r ^ 2? c) would be to find that there was no reason for there to be a privileged reference frame (ie the inertial frame of reference presents the special theory of relativity)? d) would be because the Newtonian gravitation had presupposed an action to the instantaneous distance, violating the principle of the constancy and the speed of light limit? In fact, the physics textbooks (in the area of gravity or relativity) induce the readers to believe that Einstein postulated his GR mainly due to the principle of equivalence. But that would be too simplistic!