I N T R O D U C T I O N T O V O L U M E 7 x x x i i i
between geodesic and nongeodesic motion in relativity. The metric field, however,
is a perfectly legitimate physical entity, whereas absolute space was not. As empha-
sized, for instance, in the Princeton lectures (Einstein 1922c [Doc. 71]), the metric
field acts on and is acted upon by material sources. The old ether or absolute space
acted without being acted upon. Einstein now represents this as the essence of
Mach’s objection to absolute space. In this way, Einstein is able to present the met-
ric field of general relativity as the new ether in his Leyden lecture on “Ether and
Relativity” (Einstein 1920j [Doc. 38]).
To eliminate all vestiges of absolute space, Einstein in 1917–1918 attached great
importance to what he named “Mach’s principle,” the idea that matter fully deter-
mines the metric field. The De Sitter solution, however, showed that general rela-
tivity, even with the cosmological constant, does not satisfy this requirement.
Einstein was forced to acknowledge this in the course of 1918 in correspondence
with Felix Klein (see Vol. 8, the editorial headnote “The Einstein-De Sitter-Weyl-
Klein Debate,” pp. 351–357). By the time of his Leyden lecture (Doc. 38), “Mach’s
principle” seems to have lost some of its importance for Einstein. He came to real-
ize that it is predicated on an antiquated ontology that seeks to reduce the metric
field to an epiphenomenon of matter. Matter, however, should itself be described
by fields, and Einstein gradually moved from Machian attempts to reduce one field
to another to a unified field theory where the metric field and the electromagnetic
field are both aspects of one unified field.
With Docs. 4 and 38, Einstein appears to have distilled the core conceptual foun-
dation of general relativity from his earlier statements. However, in Einstein 1918k
(Doc. 13), Doc. 31, and Einstein 1922c (Doc. 71), he returns to formulations of the
conceptual basis of general relativity that are much closer to the route by which he
arrived at the theory. This may be partly explained by Einstein’s desire to modify
his presentation to suit his audience. In the Princeton lectures (Doc. 71) he presents
the principles of general relativity in their role as heuristics that aided him in find-
ing the theory and which may aid others, particularly physicists, in understanding
it. His account of general relativity in the Princeton lectures seems, at times, to
recapitulate his own route to the discovery of the field equations. The last part of
the book is devoted to a long discussion of Mach’s principle and its role in the the-
ory (with a prominent mention of the work of Hans Thirring on what has become
known as frame-dragging) and the problem of the relativity of inertia in the light of
Einstein’s ideas on cosmology. Certainly, The Meaning of Relativity, which would
go through several more English-language editions after Einstein’s move to Amer-
ica, played a key role in introducing Mach’s principle to physicists as one of the
best-known features of Einstein’s relativity theory. The Princeton lectures illustrate
Einstein’s engagement with a variety of audiences. Of the five lectures delivered