The article develops a Kantian account of constitutional justice: the explication of those structural features of a legal system whose purpose is to optimise a polity’s capacity to achieve a Rightful condition. The People, in enacting a rights-based constitution, have placed their freedom in trust. Rights ground a system of reciprocal freedom among individuals, while conferring on officials the authority to make and enforce law, subject to constraints laid down by the Universal Principle of Right [UPR]. A constitutional court, the trustee of the regime, supervises the rights-regarding acts of all other officials, assesses the reasons officials give when they take decisions that burden rights, and invalidates those acts when reasons given to justify such burdens fail to meet the demands of the UPR. Although some rights will be expressed in absolute terms, most will be qualified by a limitation clause. In adjudicating qualified rights, the court can do no better than to adopt the proportionality principle. The UPR, operationalised through proportionality analysis, lays down a basic criterion for the legitimacy of all law. Because Public, International, and Cosmopolitan Right share certain micro-foundations in common, we can extend the analysis to transnational systems of rights protection.