Quantum dynamics is linear because quantum states are epistemic

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.