I shall conduct the reader over the road that I have myself traveled, rather a rough and winding road, because otherwise I cannot hope that he will take much interest in the result at the end of the journey. The conclusion I shall arrive at is that the field equations of gravitation which I have championed hitherto still need a slight modification.

Cosmological Considerations on the General Theory of Relativity, Proceedings of the Prussian Academy of Sciences, Volume X, 1917 (p. 142)