The standard iterative conception of sets treats sets as timeless metaphysical collections formed at successive stages; the coding conception of set reinterprets sets as finite computational codes, illuminating constructive mathematics.

Standard Zermelo-Fraenkel set theory (ZFC) rests on the 'iterative conception': sets are formed in stages from the bottom up, accumulating into a vast transfinite cumulative hierarchy.
While intuitively appealing, the metaphysical realism of the iterative hierarchy creates thorny foundational paradoxes regarding the objective existence of uncountably infinite power sets and inaccessible cardinals.
This mathematical logic paper develops the 'coding conception of set,' wherein mathematical sets are understood not as abstract metaphysical containers, but as computational codes and informational data structures encoding relational graphs.
The coding conception provides a rigorous philosophical and algorithmic foundation for constructive mathematics, proof assistant development (like Lean and Coq), and category-theoretic computer science.
The Coding Conception of Set
We propose the Coding Conception of ordinals and sets, which takes Cantor's three generating principles as its sole foundation. Bounded sets of ordinals are generated synchronously with the ordinals themselves through a bijective encoding function that, at each stage, selects only the finitely many bounded sets actually required by the successor, limit, and restriction principles. This selective coding yields the first-order theory , which we establish is the metamathematically correct theory of the ordinals: it is bi-interpretable with , yet makes no claim about the general concept of set. Extending the conception to full set theory via a monadic second-order ordinal theory with arithmetic and class comprehension produces two mutually inconsistent first-order set theories according to distinct maximality intuitions: a Type-A universe , in which the power set of every ordinal is a set and the universe satisfies ; and a Type-B universe , in which sets are strictly more than ordinals and a ``largeness cardinal'' exists, beyond which power sets remain unencodable. We prove that this Power Set Dichotomy is unavoidable, even under potentialism, and conclude that ``every cardinal has a successor'' is the only philosophically uncontroversial common fragment of any true set theory; the status of the full power-set axiom remains the sole open philosophical choice point.
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