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I'm generally aware that there are logics beside classical predicate logic—fuzzy logic, modal logic, and paraconsistent logics, just to name a few.

I appreciate whenever I learn about how some aspect of the mathematics with which I'm aware is more rigorous than I had thought, which is why I appreciate the consistency (as we've thus far seen) of ZF(C) set theory.

The question here is whether the rules of the logic in question are considered part of the axiomatic foundation: for any given consistent mathematical foundation, could we use any logic we choose in combination with any collection of set-theoretic axioms?

My confusion began with learning classical logic and predicate logic, since, although it made sense to me, it also didn't seem to have concretely stated or summarizable axioms (like ZF(C) does). As far as I can tell, logic exists to express the conditions for both the truth and validity of a logical statement, and predicate logic exists to analyze the conditions under which indeterminates within a logical statement can change the truth value (but not the validity) of those statements. I generally understand inference rules and truth tables, but I don't know if there's one formal language in which I could sequentially write each step in the process of dealing with a logical problem.

How would results formulated in ZF(C) set theory would change if the logical foundation were shifted to some other non-classical logic? What are the axioms of predicate logic? And what is the relationship between these?

For reference, I'm just finishing up my first semester of Discrete Mathematics, but you don't have to go easy on me. I'd like to know what domain of math I should research to learn more on this line of questioning.

(P.S.: I'm using "ZF(C)" to acknowledge that not everyone accepts the Axiom of Choice.)

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    $\begingroup$ I don't think this would count as an answer necessarily, but you can also have Categorical Set Theory. Which is "Set theory" but in the language of Categories and not necessarily FOL. Equivalence Diagram goes something like this: Category theory $\to$ Lambda Calculus $\to$ Hilbert Style Deduction (giving you Logic). Since we can get a Logic, it follows that we can model sets in the language of categories. $\endgroup$ Nov 14, 2023 at 23:07
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    $\begingroup$ Regarding your subquestion about the rules/axioms of predicate logic: you can rewrite/formalize any ordinary proof (the kind you encounter during a standard undergraduate degree) using the rules of the classical sequent calculus presented here along with axioms of ZFC on the left of the turnstile $\vdash$ and the statement to be proved on the right. Some form of Natural Deduction might be easier to learn instead of Sequent Calculus, and also does the job. You don't need anything else: this is an adequate formal language for the purpose. $\endgroup$
    – Z. A. K.
    Nov 15, 2023 at 1:34
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    $\begingroup$ The lack within mathematics of a unified clarity about deductive systems comes from the fact that there are lots of different deductive systems that are all equivalent to each other, and as powerful as you could want: (from A we can prove B) is equivalent to (any structure that satisfies A satisfies B). See en.wikipedia.org/wiki/G%C3%B6del%27s_completeness_theorem $\endgroup$
    – TomKern
    Nov 15, 2023 at 6:32

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Classical first order logic has many different equivalent presentations (not all of which are based on axioms!). One axiomatic variation can be found here, but really if you want to learn it you should consult an introductory mathematical logic book.

ZF(C) is very much dependent on the logic used. It is based on classical first-order logic and that controls what the logical consequences of the axioms (i.e. theorems) are. There are variations of ZF with similar axioms, but based on constructive first order logic rather than classical, but these are significantly different from ZF (and related classical set theories) in their consequences.

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  • $\begingroup$ Would you recommend the book "forallx"? $\endgroup$ Nov 14, 2023 at 22:02
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    $\begingroup$ @ToastedUranium I don't really have a favorite intro logic book to recommend. I've seen that one and don't have any good reason not to recommend it. I'd suggest trying a few if you can and going with whichever one winds up being most readable. $\endgroup$ Nov 14, 2023 at 22:05

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