Whatever exerts effects as powerful and real as the metric structure of the world exerts must itself be something real, not merely a fixed abstract structure.

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