  In this paper I describe the genesis of Einstein's early work on the problem
of motion in general relativity (GR): the question of whether the motion of
matter subject to gravity can be derived directly from the Einstein field
equations. In addressing this question, Einstein himself always preferred the
vacuum approach to the problem: the attempt to derive geodesic motion of matter
from the vacuum Einstein equations. The paper first investigates why Einstein
was so skeptical of the energy-momentum tensor and its role in GR. Drawing on
hitherto unknown correspondence between Einstein and George Yuri Rainich, I
then show step by step how his work on the vacuum approach came about, and how
his quest for a unified field theory informed his interpretation of GR. I show
that Einstein saw GR as a hybrid theory from very early on: fundamental and
correct as far as gravity was concerned but phenomenological and effective in
how it accounted for matter. As a result, Einstein saw energy-momentum tensors
and singularities in GR as placeholders for a theory of matter not yet
delivered. The reason he preferred singularities was that he hoped that their
mathematical treatment would give a hint as to the sought after theory of
matter, a theory that would do justice to quantum features of matter.
