  To date, there has been no experimental evidence that invalidates quantum
theory. Yet it may only be an effective description of the world, in the same
way that classical physics is an effective description of the quantum world. We
ask whether there exists an operationally-defined theory superseding quantum
theory, but which reduces to it via a decoherence-like mechanism. We prove that
no such post-quantum theory exists if it is demanded that it satisfy two
natural physical principles: causality and purification. Causality formalises
the statement that information propagates from present to future, and
purification that each state of incomplete information arises in an essentially
unique way due to lack of information about an environment. Hence, our result
can either be viewed as evidence that the fundamental theory of Nature is
quantum, or as showing in a rigorous manner that any post-quantum theory must
abandon causality, purification, or both.
