Christianity is heading for a full allegorization

[Angrboda]

And if we allow for multiple 'substances', is it clear that all 'mental substances' are the same? Why not many different physical substances and many different mental substances? maybe, the distinction into 'physical substances' and 'mental substances' is, itself, problematic.

The problem with multiple substances is that you quickly run into questions of how the particles manage to interact with each other if they aren't all the same substance.

These three "Worlds" are not proposed as isolated universes but rather are realms or levels within the known universe.

Their numbering reflects Popper's view (a) of their temporal order within the known universe, and (b) that each later realm emerged from developments within the preceding realm. A one-word description of each realm is that World 1 is the physical realm, World 2 is the mental realm, and World 3 is the cultural realm - though, in the detail of Popper's theory, each "World" or realm transcends what might be typically understood by the respective terms "physical", "mental" and "cultural".

In Popper's theory, there is at points direct interaction between World 1 and World 2 (at the point of mind-brain liaison), and between World 2 and World 3 (at the point where mental states are engaged with World 3 content), but there is no direct interaction at any point between World 1 and World 3 - World 3 does affect World 1 but only indirectly, through the workings of World 2 with World 3 content and then World 2's subsequent interaction with World 1.

Wikipedia || Popper's three worlds

Patricia Churchland has an elegant criticism of such three world hypotheses which I'll have to lookup. I believe that it's in Neurophilosophy in the context of talking about dualistic theories of consciousness and how they explain the interaction between the two if they don't otherwise interact with other things. Plato's Phaedrus also contains some acute reasoning on this which leads to similar conclusions in the context of whether the gods can understand man and vice versa, with the seeming conclusion that once you start postulating separate substances, a whole host of problems appear. Though to be sure, substance as a term in philosophy is sufficiently ambiguous that I suppose you could postulate any number of Rube Goldberg like solutions. Perhaps at the bottom it's more of a semantic problem than a real one, that once you have commitments to the meaning of substance, anything beyond a monism presents issues.