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Theorists like to do a lot with a little. And not just because simple theories seem more elegant: we deepen our understanding when we learn that disparate phenomena are linked together. In physics, for example, the theory of thermodynamics showed us the relationship between mechanics and heat. In economics, the theory of the firm showed us that, across industries that look nothing alike, a simple principle helps explain the organization of economic activity. Of course, there is no guarantee that the disparate phenomena we suspect are linked actually are. Particle physicists continue to search for a Grand Unified Theory, which would integrate gravity with the other fundamental forces. It may be that there is no such theory, or that we are not in a position to discover it, even if there is one. But absent such a theory, our understanding of the way the world works seems incomplete. And our track record of linking disparate physical phenomena (mechanics and heat, electricity and magnetism, space and time) gives us good reason to suppose that there are deeper explanations in the offing. Legal theorists like to do a lot with a little too, but their track record is decidedly less impressive. John Austin, for example, thought that he could capture the nature of law in a simple slogan: law is the command of the sovereign. H.L.A. Hart showed that Austin's theory was too simple. Among other problems, some laws are not commands, and some legal systems do not have sovereigns, at least in Austin's sense. But Hart had a simple theory of his own: a legal system is a union of primary and secondary rules. Alas, Hart's theory was too simple too. It didn't distinguish law from other systems with primary and secondary rules (chess, for example, or a university's regulations). And there are reasons to doubt that rules are the fundamental building blocks of law, or that law even has fundamental building blocks at all. Indeed, it is possible that the connections between the things we regard as legal systems are simply historical, such that there is nothing - or at least nothing significant - that all of them share in common. To make this point, Ronald Dworkin invoked Wittgenstein's image "of a rope composed of many strands no one of which runs for its entire length or across its entire width." If law is like that, then the search for a common thread that runs through it, wherever and whenever it is found, is doomed to fail.