Dear Jochen,
thanks for your reply. The time you take for this is highly appreciated!
Before going into details of what you write, I think that the answer to many of your comments is: it depends on what we mean by "structure". In my essay, I'm not really defining it properly (in particular in the section on the physical world). Any serious philosophical approach that tries what I've sketched will have to give a clear(er) definition. Also OSR has to do that (and I suspect that it does, but they mean something slightly different from what I mean).
Regarding your example of the Maxwell's equations and String Theory, I think that what this tells us is that we should define "structure" in terms of "big enough equivalence classes", or admit "coarse enough isomorphisms" when we define it. In particular, there may be two theories T and T' that talk about the same structure S. I would say that your example of Electromagnetism (i) and String Theory (ii) is of that kind: two theories that give the *same* structure.
Being a structural realist in *that* sense, I'd say that there is no ontological difference between the statements that "(i) is true" and that "(ii) is true". But I admit that this does not really come across in my essay, because I'm not giving a clear-enough definition of "structure". And there are many questions that such a view leaves open.
You are right that it would be nice to talk more to Karl Svozil; I've only met and chatted with him once, and he seems to have many clever ideas and insights that touch these topics.
But I'd also find it nice to meet you in person at some point in the near future. Perhaps I can invite you to visit IQOQI when the Corona crisis is over? It would be a lot of fun to chat in person! Also, we have a regular Physics-Philosophy-Meeting here that you might enjoy.
Now, regarding your second email and quantum mechanics:
Again, I was clearly not detailed enough to say what I mean by structure or differentiation in this context. That is certainly a drawback of my essay (also due to space limits, of course, but also I don't really know how to do the definitions properly -- it's more an idea than something fully worked out.)
Here's what I do *not* mean. I don't mean to say that the world now is structure S, and once we learn an additional measurement outcome, it evolves into a more differentiated structure S'.
Instead, consider the quantum world on all of spacetime. There are certain "real patterns" in accordance with quantum physics: for example, certain events that happen earlier on (perhaps "preparations") are in correspondence with frequencies of certain types of events later on (correlations with "measurement outcomes"). This would be structure S.
A more differentiated structure S' would be the world according to de Broglie-Bohm theory: additional (unobservable) events earlier on that are deterministically correlated with outcomes later on. The corresponding theory T' makes more claims than T, and so S' would be more differentiated than S.
In other words: I'm just saying that views in which quantum probabilities are not "knowledge *about* the world" are in some structural sense less differentiated than view in which they are. So a structural view may increase one's confidence to accept views of the former kind.
I'm really curious to find out about your "middle way", but I'll postpone commenting on it until I have read your essay. I'm having a busy time with some deadlines next week, but I'm eager to read your essay directly after that.
Best,
Markus