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It is natural to expect a complete physical theory to have the ability toconsistently model agents as physical systems of the theory. In [Nat. Comms. 9,3711 (2018)], Frauchiger and Renner (FR) claim to show that when agents inquantum theory reason about each other's knowledge in a certain Wigner's friendscenario, they arrive at a logical contradiction. In light of this, Renneroften poses the challenge: provide a set of reasoning rules that can be used toprogram quantum computers that may act as agents, which are (a) logicallyconsistent (b) generalise to arbitrary Wigner's friend scenarios (c)efficiently programmable and (d) consistent with the temporal order of theprotocol. Here we develop a general framework where we show that every logicalWigner's friend scenario (LWFS) can be mapped to a single temporally orderedquantum circuit, which allows agents in any LWFS to reason in a way that meetsall four criteria of the challenge. Importantly, our framework achieves thisgeneral resolution without modifying classical logic or unitary quantumevolution or the Born rule, while allowing agents' perspectives to befundamentally subjective. We analyse the FR protocol in detail, showing how theapparent paradox is resolved there. We show that apparent logicalcontradictions in any LWFS only arise when ignoring the choice of Heisenbergcut in scenarios where this choice does matter, and taking this dependence intoaccount will always resolve the apparent paradox. Our results establish thatuniversal applicability of quantum theory does not pose any threat tomulti-agent logical reasoning and we discuss the implications of these resultsfor FR's no-go theorem. Moreover, our formalism suggests the possibility of atruly relational and operational description of Wigner's friend scenarios thatis consistent with quantum theory as well as probability theory applied tomeasurement outcomes.

A general framework for consistent logical reasoning in Wigner's friend scenarios: subjective perspectives of agents within a single quantum circuit