Abstract: This paper reconstructs the foundation of legal philosophy by dismantling the Kantian categorical imperative and redefining normativity through the concept of the "constitutive hypothetical imperative." It first demonstrates that the categorical imperative is an illusion rooted in reason's conflation of analysis and decision. Drawing on Weber's typology of social action, it argues that value-rationality (Wertrationalität) is the primordial motive of all action, and it redefines reason as the faculty of cognition and the will as the autonomous faculty of decision, thereby dissolving the spurious redundancy of the will in Kant's framework. The paper then locates the genesis of law in the transcendental conditions of the intersubjective field: drawing on Fichte's insight that the I posits itself only through opposition to a not-I, it argues that the self is differentially constituted through the other and therefore necessarily wills to maintain the integrity of that field. Law is consequently a constitutive hypothetical imperative: "If you will to maintain the field of intersubjectivity, you ought to obey the law." This imperative unfolds through the distinction between nomos—the transcendental, formal, universal, and compound (negative and positive) minimal conditions for the field's integrity—and lex—the positive, procedural, and institutional expression of nomos within a given community. Finally, the paper distinguishes law from ordinary empirical rules and critiques legal positivism, exemplified by Hart's union of primary and secondary rules, for reducing law's normativity to social convention. Instead, it recasts lex as a "meta-rule," whose binding force derives not from sanctions or convention, but from the constitutive presupposition that makes selfhood possible. The normativity of law is thus neither moral metaphysics nor mere coercion, but the self's pre-existing practical commitment to the intersubjective conditions of its own differential establishment. Keywords: categorical imperative; constitutive hypothetical imperative; valuerationality; intersubjectivity; nomos and lex; legal positivism; Hart; Fichte; Kant; selflegislation