Is the world quantum? An active research line in quantum foundations is devoted to exploring what constraints can rule out the postquantum theories that are consistent with experimentally observed results. We explore this question in the context of epistemics, and ask whether agreement between observers can serve as a physical principle that must hold for any theory of the world. Aumann’s seminal Agreement Theorem states that two observers (of classical systems) cannot agree to disagree. We propose an extension of this theorem to no-signaling settings. In particular, we establish an Agreement Theorem for observers of quantum systems, while we construct examples of (postquantum) no-signaling boxes where observers can agree to disagree. The PR box is an extremal instance of this phenomenon. These results make it plausible that agreement between observers might be a physical principle, while they also establish links between the fields of epistemics and quantum information that seem worthy of further exploration.

世界是量子的吗？量子基础中的一条活跃研究线致力于探索哪些约束可以排除与实验观察结果一致的后量子理论。我们在认识论的背景下探索这个问题，并询问观察者之间的一致性是否可以作为任何世界理论都必须遵守的物理原理。奥曼开创性的一致性定理指出，（经典系统的）两个观察者不能同意不同意。我们建议将此定理扩展到无信号设置。特别是，我们为量子系统的观察者建立了一个协议定理，同时我们构建了（后量子）无信号盒的例子，观察者可以在其中同意不同意。PR 框是这种现象的一个极端实例。