In Einstein's theory of gravitation matter and its dynamical interaction are based on the notion of an intrinsic geometric structure of the space-time continuum. The ideal aspiration, the ultimate aim, of the theory is not more and not less than this: A four-dimensional continuum endowed with a certain intrinsic geometric structure, a structure that is subject to certain inherent purely geometrical laws, is to be an adequate model or picture of the ‘real world around us in space and time’ with all that it contains and including its total behaviour, the display of all events going on in it.

Indeed the conception Einstein put forward in 1915 embraced from the outset (and not only by the numerous subsequent attempts to generalize it) every kind of dynamical interaction, not just gravitation only. That the latter is usually in the foreground of our mind—that we usually call the theory of 1915 a theory of gravitation—is due to two facts. First, its early great successes, the new phenomena it predicted correctly, were deemed to refer essentially to gravitation, though that is, strictly speaking, true only for the precession of the perihelion of Mercury. The deflexion of light rays that pass near the sun is not a purely gravitational phenomenon, it is due to the fact that an electromagnetic field possesses energy and momentum, hence also mass. And also the displacement of spectral lines on the sun and on very dense stars (‘white dwarfs’) is obviously an interplay between electromagnetic phenomena and gravitation.