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This article illuminates the largely misunderstood relationship between complexity and the regulation of property interests Specifically the article presents the complexity thesis a novel explanatory account of the principle of numerus clausus The principle of numerus clausus is an ancient common law rule which prohibits the customization of property interests The complexity thesis holds that the primary function of numerus clausus is to prevent the proliferation of highly idiosyncratic property interests In so doing numerus clausus provides a bulkhead against the overwhelming complexity that would ensue if customized property interests were permitted For the last fifteen years numerus clausus has been the subject of a spirited colloquy in which property theorists of all methodological stripes have sought to unravel some of the mysteries that surround the principle This article carefully engages several prominent explanatory accounts of numerus clausus and demonstrates that while these competing accounts supply a number of important insights about the principle the complexity thesis does a better job of accounting for all of the salient features of the principle without sacrificing coherence or consilience Finally the complexity thesis is especially instructive today as the 2007 collapse of the housing market can largely be traced to a set of basic misapprehensions about the destructive power of complexity in the context of highly alienable interests The complexity thesis demonstrates that standardization serves an essential epistemic function Standardization makes it possible for us to better apprehend risk and thereby avoid catastrophic miscalculations such as those that led to the housing collapse