  I now come to the crucial idea of the theory of General Relativity.  Whatever exerts as powerful and real effects as does the metric structure of the world, cannot be a rigid, once and for  all, fixed geometrical structure of the world, but must  itself be something real which not only exerts effects on  matter but which in turn suffers them through matter. Riemann  already suggested for space the idea that the structural field, like  the electromagnetic field, reciprocally interacts with matter.