 Price (2012) turns these sketchy considerations into a rigorous argument. He shows that, when combined with two assumptions concerning quantum ontology, time-symmetry implies retrocausality (understood along broadly interventionist lines). The ontological assumptions are (i) that at least some aspects of the quantum state \(\psi\) are real (notably, in Price’s example, there is a “beable” encoding photon polarization angle), and (ii) that inputs and outputs of quantum experiments are discrete emission and detection events. Moreover, it is important to Price’s argument that dynamical time-symmetry (that the dynamical laws of the theory are time-symmetric) be understood as implying that operational time-symmetry (that the set of all possible combinations of preparation and measurement procedures in a theory, with associated probabilities for outputs given inputs, is closed under interchange of preparation and measurement) translates into ontic time-symmetry (operational time-symmetry plus a suitable map between the ontic state spaces of the symmetric combinations). Given these conditions, any foundational account that reproduces the empirical verdicts of quantum theory must be retrocausal.