  The dynamics of general relativity is encoded in a set of ten differential
equations, the so-called Einstein field equations. It is usually believed that
Einstein's equations represent a physical law describing the coupling of
spacetime with material fields. However, just six of these equations actually
describe the coupling mechanism: the remaining four represent a set of
differential relations known as Bianchi identities. The paper discusses the
physical role that the Bianchi identities play in general relativity, and
investigates whether these identities -- qua part of a physical law --
highlight some kind of a posteriori necessity in a Kripkean sense. The inquiry
shows that general relativistic physics has an interesting bearing on the
debate about the metaphysics of the laws of nature.
