This paper presents an axiomatization of Ludwig von Mises' praxeology in many-sorted first-order logic, isolating the foundational layer. We introduce a formal language with five sorts (Actors, Actions, Ends, Things, Times) and six primitive relations (Acts, Avail, EndOf, Use, a preference order, and a time order), together with a base axiom system organized into three layers: the structure of action itself, the actor's preference order together with its demonstration in choice, and material scarcity. The base system captures purposeful action in its bare praxeological form. Working entirely within the base system we derive the core classical Misesian propositions as Hilbert-style theorems: the asymmetry of demonstrated preference, the existence of opportunity cost, the structural scarcity of time, the subjectivity of opportunity cost, the law of diminishing marginal utility, and the increasing marginal disutility of labor. Where a theorem requires structure beyond the praxeological core, as with diminishing marginal utility, the additional premises are made explicit; identifying these hidden premises is one of the methodological payoffs of the approach. A self-contained Lean 4 companion encodes the language as Lean 4 type classes and constructs a concrete infinite-time Robinson Crusoe model whose acceptance by the type-checker is a constructive consistency proof of the full base theory.