  According to quantum theory, a scientist in a sealed laboratory cannot tell
whether they are inside a superposition or not. Consequently, so long as they
remain isolated, they can assume without inconsistency that their measurements
result in definite outcomes. We elevate this to the status of a general
principle, which we call Local Definiteness. We apply this principle in the
context of modifications of quantum theory that allow the dynamics to be
non-linear. We prove that any such theory satisfies Local Definiteness if and
only if its dynamics is linear. We further note that any interpretation that
takes quantum states to be epistemic necessarily satisfies the principle,
whereas interpretations that take quantum states to be ontic do not satisfy it,
unless they make additional assumptions that amount to presupposing linearity
of the dynamics. Therefore the reason why experiments to date have not found
evidence of non-linear dynamics might simply be that quantum states are
epistemic.
