Can you say what you mean by ontological indeterminacy? Do you have any examples?
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Ontological (in)determinism concerns (in)determinate state changes in complex systems, independently of how different categories of observers may (or may not) be able to acquire knowledge about these changes.
Epistemological (in)determinism, on the other hand, characterizes systems as (non)determinable by idealized observers, who may (not) be able to obtain complete and accurate information about the systems present state, in order to convert such into data about future state(s) by using the appropriate dynamical laws."
Indeterminacy: The Mapped, the Navigable, and the Uncharted
Translation- ontological indeterminacy involves unpredictably that cannot become predictable even with ideal observers and measurements. That is, even in theory we couldn't know future states. Ontological means it "really exists". Epistemic indeterminacy, on the other hand, concerns practical limits. We can't "know" future states because there are just too many variables or processes for us to handle now, but if we had perfect instruments and ideal observers, then we could know these states. In the former case, we are limited because the unpredictability is part of the system itself. This need not involve random (in the probabilistic sense) processes like in QM. Nonlinear systems present a challenge because our approach to nonlinearity mathematically involves treating curves like lines. Locally, they are like lines. However, in higher dimensions the phase space of complex systems behave so erratically that even infinitesimal changes in time lead to fluctuations we can't even in theory model. Such systems are ontologically indeterminate in that the necessary solutions are impossible (the systems are mathematically intractable). Epistemic indeterminacy can be quite similar to this type of ontological indeterminacy. The difference is subtle, but a simplistic and easier way of conceptualizing it is to think of complex systems which are ontologically indeterminate as being too complex for mathematical models to "solve" while epistemic indeterminacy involves too many processes and external influences for use to control (or control for) mechanically in e.g., an experiment.
My take is that "too complex" and unknown variables will always be a factor in determining or not determining future states.
Quantum mechanics is a statistical mechanics at heart. It is entirely deterministic only because it treats the system as being in multiple states at once. There is no way of knowing which one of (potentially infinite) possible states one will get without measurement. The indeterminacy is absolute and we cannot even in theory "know" future states. Currently, although some postulate that there are other, similar processes that are absolutely-even-in-theory-indeterminate we have no empirical evidence that these aren't just too hard to model mathematically or too difficult for us to work with experimentally rather than exhibiting the kind of randomness inherent in QM.