I'm far from being an expert in the field of mathematical logic, but I've been reading about the academic work invested in the foundations of mathematics, both in a historical and objetive sense; and I learned that it all seems to reduce to a proper -axiomatic- formulation of set theory.
It also seems that all set theories (even if those come in ontologically different flavours, such as the ones which pursue the "iterative approach" like ZFC, versus the "stratified approach" -inspired by Russell's and Whitehead's type theory first formulated in their Principia- such as Quine's NFU or Mendelson's ST) are built as collections of axioms expressed in a common language, which invariably involves an underlying first order predicate logic augmented with the set-membership binary relation symbol. From this follows that FOL makes up the (necessary) "formal template" in mathematics, at least from a foundational perspective.
The justification of this very fact, is the reason behind this question. All the stuff I've read about the metalogical virtues of FOL and the properties of its "extensions" could be summarized as the statements below:
- FOL is complete (Gödel, 1929), compact and sound, and all its particular formalizations as deductive systems are equivalent (Lindström, 1969). That means that, given a (consistent) collection of axioms on top of a FOL deductive system, the set of all theorems which are syntactically provable, are semantically satisfied at least by one model of the axioms. The specification of the axioms absolutely entails all its consequences; and the fact that every first order deductive system is equivalent, suggests that FOL is a context-independent (i.e. objective), formal structure.
- On the other hand, the Löwenheim–Skolem theorem implies that FOL cannot categorically characterize infinite structures, and so every first order theory satisfied by a model of a particular infinite cardinality, is also satisfied by multiple additional models of every other infinite cardinality. This non-categoricity feature is explained to be caused by the lack of expressive power of FOL.
- The categoricity results that FOL-based theories cannot achieve, can be obtained in a Second Order Logic (SOL) framework. Examples abound in ordinary mathematics, such as the Least Upper Bound axiom, which allows the definition of the real number system up to isomorphism. Nevertheless, SOL fails to verify an analog to the completeness results of FOL, and so there is no general match between syntactic provability and semantic satisfiability (in other words, it doesn't admit a complete proof calculus). That means that, even if a chosen collection of axioms is able to categorically characterize an infinite mathematical structure, there is an infinite set of wff's satisfied by the unique model of the axioms which cannot be derived through deduction.
- The syntactic-semantic schism in SOL also implies that there is no such a thing as an equivalent formulation of potential deductive systems, as is the case in FOL and stated by Lindström's theorem. One of the results of this fact is that the domain over which second order variables range must be specified, otherwise being ill-defined. If the domain is allowed to be the full set of subsets of the domain of first order variables, the corresponding standard semantics involve the formal properties stated above (enough expressive power to establish categoricity results, and incompleteness of potential, non-equivalent deductive systems). On the other hand, through an appropiate definition of second order domains for second order variables to range over, the resultant logic exhibits nonstandard semantics (or Henkin semantics) which can be shown to be equivalent to many-sorted FOL; and as single-sorted FOL, it verifies the same metalogical properties stated at the begining (and of course, its lack of expressive power).
- The quantification extension over variables of successive superior orders can be formalized, or even eliminate the distinction between individual (first order) variables and predicates; in each case, is obtained -for every N- an Nth Order Logic (NOL), and Higher Order Logic (HOL), respectively. Nevertheless, it can be shown (Hintikka, 1955) that any sentence in any logic over FOL with standard semantics to be equivalent (in an effective manner) to a sentence in full SOL, using many-sorting.
- All of this points to the fact that the fundamental distinction, in logical terms, lies between FOL (be it single-sorted or many-sorted) and SOL (with standard semantics). Or what seems to be the case, the logical foundations of every mathematical theory must be either non-categorical or lack a complete proof calculus, with nothing in between that trade-off.
Why, so, is FOL invariably chosen as the underlying logic on top of which the set theoretical axioms are established, in any potentially foundational formalization of mathematics?
As I've said, I'm not an expert in this topic, and I just happen to be interested in these themes. What I wrote here is a summary of what I assume I understood of what I read (even though I'm personally inclined against the people who speaks about what they don't fully understand). In this light, I'd be very pleased if any answer to this question involves a rectification of any assertion which happened to be wrong.