Of universes and worlds and stuff
I want to discuss how "universe", "world" and "thing" (or whatever word is chosen instead; for instance "object", "entity", "item", etc.) are best defined in philosophical contexts. Note that. Philosophical contexts. It is irrelevant how science uses "universe" as in, for instance, multi-verses. I have made some preliminary research in form of definition proposals. I want to put them to the test. I am only interested in analytic research. If you're interested in some vague discussion of similar question, then create a new thread for that. I assume that participants in this thread has some knowledge of logic, especially propositional logic and predicate logic. (And perhaps alethic logic too.)
Definition ideas

Some ideas about "universe":



U1. Universe ≡ the container of all contingent things.
U2. Universe ≡ that which all contingent things are a part of.
U3. Universe ≡ the set of all contingent things.



Some ideas about "world":



W1. World ≡ the container of all things.
W2. World ≡ that which all things are a part of.
W3. World ≡ the set of all things.



Thoughts and questions for further research

I consider it likely that it is a good idea to define "universe" and "world" in a likewise manner. If we do so, then the universe is in some sense a 'subset' of the world. One question is this: Is an empty universe possible (meaning logically possibility, that is, self-consistency)? Is an empty world? Is the universe a thing? Is it contingent? If yes to both questions, then the universe is part of itself. Is that a problem? Are there similar problems with the world? If we want to define "thing" (or whatever word) in such a way as to exclude the world, the universe or both, then how do we do that?



There are some problems with (3)-type definitions, and that is Russell's paradox. I don't understand the mathematics completely.