DOC.
15
THEORY OF GRAVITATION
191
z,
t
are
referred)
in
such
a
way
that
only
linear
but,
in
contrast to
the
customary
theory
of
relativity,
arbitrary
linear substitutions leave the
equations
covariant.
With
these restrictions
on
the reference
system, one
attains
completely
determinate
equations
of
gravitation
that
satisfy
all
of the conditions that
we
may impose
on
the
gravitational equations.
In
particular,
the
equations
entail the
conception
that the inertia
of
bodies is not
a
property
of individual accelerated bodies
by themselves,
but rather
an
interaction,
i.e.,
a
resistance
to
the relative acceleration of bodies with
respect
to
other
bodies-a
conception
that has
already
been advanced
by
Mach and others
on
epistemological
grounds.