  Following Frauchiger and Renner's discovery of a conflict between quantum
mechanics and certain commonsense reasoning axioms, much work has gone into
finding alternative axiomatizations that can avoid the conflict. However, this
body of work is largely based on specific interpretations of quantum mechanics,
and at times employs specialized formalism that may be inaccessible to
mainstream quantum information experts. Taking an interpretation-agnostic
approach, we propose a simple operational principle called superpositional
solipsism to aid rational agents in making situational inferences. We show that
the principle leads to sound inferences in all operationally-relevant
instances. Along the way, we discuss certain subtleties about the
Frauchiger--Renner result that may have gone hithertofore unnoticed.
