The Legitimacy of (Some) Federal Common Law
Much of the modern debate over federal common law starts from the premise that when courts articulate rules of decision as a matter of unwritten law, they are "making law" in a robust, quasi-legislative sense. For people who deny that federal courts have inherent authority to invent their own rules of decision, the logical conclusion is simple: federal common law is legitimate only where the Constitution or Congress gives the federal courts a special delegation of lawmaking power.
That conclusion, however, does not fit our legal system very well. On the one hand, it may discourage courts from recognizing even widely accepted principles of common law in enclaves of federal preemption. On the other hand, the need to recognize some unwritten law in those enclaves may make judges strain to read federal statutes or constitutional provisions as implicitly delegating quasi-legislative power to the federal courts. Where federal judges think they have such power, moreover, they may end up indulging their own policy preferences at the expense of other sources of unwritten law. By the same token, the arguments advanced by some skeptics of federal common law may have the ironic effect of encouraging state judges to think that they are completely in charge of the unwritten law of their state and can legitimately articulate whatever rules they like.
The root of all these problems lies in the skeptics' failure to distinguish rules of decision that courts make up out of whole cloth from rules of decision that reflect pre-existing sources (such as widespread customs or the collective thrust of precedents from the courts of the fifty states). While courts certainly shape even the latter sort of rules, and while the process by which courts do so can be described as a species of "lawmaking," it is not the type of lawmaking in which legislatures engage, and it might not require the sort of delegation that skeptics of federal common law have in mind.