This article takes the origin of the Universal Dividend for the era of artificial intelligence, as a justiciable constitutional grounded in existing property doctrine, not as a redistributive policy contingent on political choice. The reconstruction turns on a doctrinal correction: large-scale training data, treated by the contemporary digital economy as res nullius, is properly classified as res communes omnium—the Roman category of things common to all by natural law. The classification is established through the Justinianic source, its Grotian reception, the instructive failure of the twentieth-century common heritage of mankind regime, and the parallel failure of the cyberlibertarian commons identified by Boyle's Second Enclosure Movement. The classical category requires only narrow adjustment to accommodate a use-pattern the Roman jurists did not anticipate, non-rivalry of the underlying good combined with rivalry of the rents extracted from it, and that adjustment takes the form of a rent-capture mechanism, the Data Royalty, that does not enclose the commons but compensates its beneficial owners for monopolistic extraction. The reclassification is the doctrinal expression of the Lockean proviso, which excludes appropriation that does not leave equivalent appropriability for others. Once the doctrinal foundation is established, the article specifies the procedural instrument through which the citizen's beneficial ownership becomes enforceable: a Habeas Data of Subsistence, defined by a Minimum Viable Record of five fields—contract identity, calculation basis, adjustment log, purchasing-power index, and recourse endpoint—and operating as a general guarantee of algorithmic due process, structurally insulated. The Universal Dividend is thus reconstructed as a justiciable right whose foundation is older than the technologies that have made its enforcement urgent.