Observers as Primitives

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Let us suppose that we are trying to build a physical theory of the universe, in order to do so, we have to introduce some primitive notions, on which the theory will be based upon. We explore possible candidates that can be considered to be such "primitives": for example, the structure of the spacetime, or quantum states. However, the examples can be given such that show that these notions are not as objective as we would want them to be. The concept of objectivity, on the other hand, is closedly linked to that one of "an observer", thus, we can at least assign it as a primitive of the theory. Now agents are themselves physical systems, and we should take this into account when we specify the ground rules of what they can do. On the one hand, we take agents and their communication as a primitive of the theory and then see which concepts can be derived from there. On the other hand, we treat agents as quantum systems themselves and investigate what kind of logic applies to their interpersonal reasoning; for that, as a guiding example we use the Frauchiger-Renner thought experiment {1,2}.