  Moreover, it seems entirely implausible to assume that some operators — sums and products of operators that are associated with well-defined observables — are themselves not associated with well-defined observables, even if they mathematically inherit exact values from their summands or factors. Put in a crude example, this would amount to saying that to ask for a system’s energy is a well-defined question, while to ask for the square of the system’s energy is not, even if, from our answer to the first question and trivial mathematics, we have a well-defined answer at hand. There seems no good a priori reason to justify this restriction. So, to make rejecting VR plausible at all, an additional proposal is made: It is crucial to the KS argument that one and the same operator is constructed from different maximal ones which are incompatible: f(Q) is identical to g(P), where PQ − QP ≠ 0. We now assume that only the construction of f(Q) via Q, but not the one via P, leads to a well-defined observable in a certain  context.[14]