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The Foundation of the General Theory of Relativity
ANNALEN Der Phisik 1916

14. The Field Equations of Gravitation in the Absence of Matter

We make a distinction hereafter between " gravitational field " and " matter " in this way, that we denote everything but the gravitational field as " matter." Our use of the word therefore includes not only matter in the ordinary sense, but the electromagnetic field as well.
Our next task is to find the field equations of gravitation in the absence of matter.