Bourbaki--Zorn Normal Forms for Maximality Arguments

Abstract

We isolate a normal-form mechanism underlying Bourbaki--Witt fixed-point arguments and least-upper-bound versions of Zorn-type maximality principles. Given a progressive self-map on a partially ordered set, we define a Bourbaki tower as a well-ordered trajectory whose successor stages are generated by the map and whose limit stages are given by least upper bounds of earlier stages. We prove that least upper bounds for nonempty well-ordered subsets are sufficient to force a fixed point for every progressive self-map. Thus the fixed-point statement is obtained under a weaker completeness hypothesis than the usual chain-complete form of the Bourbaki--Witt theorem. The proof proceeds by constructing a largest Bourbaki tower. The least upper bound of this largest tower belongs to the tower itself and is a fixed point of the map. As a consequence, strictly progressive self-maps cannot exist in such posets. Combining this obstruction with a choice selector on strict upper cones yields a concise maximality principle: if every nonempty well-ordered subset has a least upper bound, then the poset has a maximal element. The contribution is methodological rather than axiomatic. The paper makes explicit a reusable proof architecture connecting well-ordered Bourbaki--Witt fixed points, strict progression obstructions, and least-upper-bound versions of Zorn-type maximality arguments.