"Special relativity has this in common with Newtonian mechanics: The laws of both theories are supposed to hold only with respect to certain coordinate sys­tems: those known as "inertial systems. " An inertial system is a system in a state of motion such that "force-free" material points within it are not accelerated with respect to the coordinate system. How­ever, this definition is empty if there is no independent means for recognizing the absence of forces. But such a means of recognition does not exist if gravitation is considered as a "field.""