  One of the most striking features of quantum theory is the existence of
entangled states, responsible for Einstein's so called "spooky action at a
distance". These states emerge from the mathematical formalism of quantum
theory, but to date we do not have a clear idea of the physical principles that
give rise to entanglement. Why does nature have entangled states? Would any
theory superseding classical theory have entangled states, or is quantum theory
special? One important feature of quantum theory is that it has a classical
limit, recovering classical theory through the process of decoherence. We show
that any theory with a classical limit must contain entangled states, thus
establishing entanglement as an inevitable feature of any theory superseding
classical theory.
