In order to solve the problem of time for constrained Hamiltonian mechanics, i.e. to consistently describe time evolution in totally constrained systems, Rovelli and collaborators make use of the framework of complete and partial observables. Totally constrained systems arise in general relativistic contexts, and so successful application of this framework is an important precursor to the quantization of gravity. In this talk, I explain how this strategy works for a simple mechanical system, illustrating the relational nature of the gauge invariant complete observables. There is then some ambiguity as to the ontological status of the gauge dependent partial observables, which are claimed to be measurable but not predictable. I advocate a two-tier ontology as an interpretive strategy, showing how the, usually general, philosophical doctrines of operationalism and functionalism can be applied piecewise to the framework, giving a more detailed picture of the relationships between mathematical formalism and actual or possible measurement outcomes.
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