Imagine a liar, who always lies. If they say “this sentence is a lie,” what happens if they are lying? Then it would mean the sentence is not a lie, but that would render the original statement false! And someone who intentionally says a false statement is …
Just to relate back to the discussion before. I believe that the Buddha claimed that he knew all things to be empty because he knew that any valid referent would have to fall within the sense spheres. What he called “the all” or “the world.” I believe that he did not comment on what is “beyond” or “not beyond” or both or neither of that, because it would be groundless. Those statements and assertions would also just be grounded in sense experience, or ‘contact.’
Photons and electrons and so on are things people talk about after experiencing data and other sensory phenomena. So if the Buddha found that all sense experience is dependently arisen and empty, then it would automatically include the whole world in his definition of the term. This is why I said before that interpretations of quantum mechanics, if they had a bearing on emptiness, would be problematic: they would undermine the foundation of the Buddha’s assertions on it. It would entail a new set of epistemological and ontological assertions, and it would seem to require some kind of omniscience, which is arguably impossible.
This is also why I don’t think there is a true distinction in early Buddhism between “selflessness” and “emptiness.” A distinct form of ‘selflessness’ is really just a reification of the senses or aggregates, what @josephzizys calls the “fictionalist” argument, I believe. But now it’s tangential. As for the related question:
Say someone rejects Scientism. They say that scientists who make measurements and so on of the physical world are unable to honestly claim they have seen ultimate reality or what the whole world is made of, such as Physicalism. They say that those are philosophical claims and questions that are not confirmed or denied by scientific research. So say they propose an instrumentalist application of science, where it is just science without ontological commitments.
Then they go to Abhidhamma class, and suppose they experience flashing particles in meditation. If they claim to have ‘seen ultimate reality’ which is irreducible or independent and existent apart from all perceivers, do you see how that would be the same category error as scientism? Just that rather than modern physics, it’s Medieval metaphysics manuals + meditation, and rather than physicalism its some kind of atomism or classification of “ultimate realities.” That too is just philosophical speculation without recognizing it. No “ultimate realities” have been proven. All that’s likely happened is that someone had some confirmation bias of Medieval metaphysics manuals based on a temporary meditation experience.