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Here is Dirac's argument about one reason to motivate quantum mechanics:

"It is usually assumed that, by being careful, we may cut down the disturbance accompanying our observation to any desired extent. The concepts of big and small are then purely relative and refer to the gentleness of our means of observation as well as to the object being described. In order to give an absolute meaning to size, such as is required for any theory of the ultimate structure of matter, it becomes necessary to assume that there is a limit to the fineness of our powers of observation and the smallness of the accompanying disturbance – a limit which is inherent in the nature of things and can never be surpassed by improved technique or increased skill on the part of the observer."

And next paragraph on causality:

"A consequence of the preceding discussion is that we must revise our ideas of causality. Causality applies only to a system which is left undisturbed. If a system is small, we cannot observe it without producing a serious disturbance and hence we cannot expect to find any causal connexion between the results of our observations. Causality will still be assumed to apply to undisturbed systems and the equations which will be set up to describe an undisturbed system will be differential equations expressing a causal connexion between conditions at one time and conditions at a later time. These equations will be in close correspondence with the equations of classical mechanics, but they will be connected only indirectly with the results of observations. There is an unavoidable indeterminacy in the calcu- lation of observational results, the theory enabling us to calculate in general only the probability of our obtaining a particular result when we make an observation."

Is Dirac already signaling(?), asserting(?) that QM is to be epistemic? How else do I buy the argument that because humans/human-sized-experiments have investigative limits, that reality must have a notion of size and smallness. Isn't the observation that our measurements cause disturbances at most a claim about what we can investigate, and not about what is? Maybe the photon is a constituent particle after all.

To me, his argument must either be epistemic or circular. There could be a smaller reality beyond the photon and quantum mechanics would still describe what we see right?

But then why is this a new kind of epistemism requiring us to modify our notion of causality? Surely scientists have always limited their theories to what we can observe? Or was physics ontic to Dirac until this point? Is the uniqueness of quantum mechanic's epistemism that it is part of the physical dynamics? The measurements cause a disturbance. Not just that we are limited to epistemic theories, but by measuring we are partaking in an epistemic physical theory/reality? Did we never have other physical/dynamical epistemic theories until the 1930s? And if there were, why didn't they need to revise our notion of causality too?

Is Dirac arguing quantum mechanics is epistemic not because we are limited to epistemic knowledge as all of science is, but quantum mechanics itself along with our measurements creates some kind of epistemic reality through a physical process involving measurement and collapse of the wavefunction? Is there even a difference between the two ideas of epistemism? Or he is arguing somehing slightly different?

I do not expect an answer to explicitly answer all of my rambling questions. Thanks

https://archive.org/stream/1quantumMechanicsByPamDirac/1quantum%20mechanics%20by%20pam%20dirac_djvu.txt

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    $\begingroup$ This seems more suitable for Philosophy SE. The "disturbance interference" interpretation of QM was also suggested by Bohr originally, but it became clear that it can not explain non-classical correlations and the standard Copenhagen interpretation discarded it. By itself, observer's interference with results of observations is perfectly compatible with classical physics, and there is no "unavoidable indeterminacy" contrary to what Bohr and Dirac thought, so no need for epistemism. That comes from more subtle features of QM formalism. $\endgroup$ – Conifold Feb 22 at 22:26

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