Legal positivism, in the most basic sense, holds that laws are the manifestation of sovereign commands. It stands in contradistinction to natural law or the judicial conformity with human reason that supposedly defines the common law.[i] Legal positivism generally rebuffs the premise that law and morals are necessarily or even customarily united. Legal positivists from Jeremy Bentham to John Austin to H.L.A. Hart maintained or implied that the formal source of the law was human promulgation, not nature or divine decree; theirs was an analytical jurisprudence that treated the normative function of the law as imposing rules and duties upon the subjects of the sovereign. Positivism generally holds that law is logical and analytical and made up of legislative policies with a linear history that can be understood through utilitarian calculation. To comprehend the law in the positivist paradigm requires analyzing the signification of words as grammatical imperatives—as “commands,” in Austin’s lexicon.
The common law, on the other hand, traditionally was seen as the vast accumulation of judicial decisions as against the commands of legislatures or the unbinding whims of equity courts; a legislative code announces rules whereas judicial decisions follow, clarify, and sustain them. The common law is a body of cases, a growing organism representing the general rules and inherited customs of the jurisdiction. It is simultaneously conservative and progressive. It comes together over time as innumerable judges and justices struggle with and against precedent to apply longstanding rules to new and unique situations. It responds and reacts to cultural norms rather than making them.
What distinguishes the common law from a civil law system is the doctrine of stare decisis (“let the decision stand”), which requires judges to follow precedents established by prior decisions or to distinguish the facts of new cases from the facts of previous cases in order to reach an applicable rule. Certain rules persevere because they triumph over lesser practices that have not worked. Oliver Wendell Holmes, Jr., explained that this process of creating and sustaining laws in graduated stages does not always make sense or produce the perfect outcome: “In form its growth is logical. The official theory is that each new decision follows syllogistically from existing precedents. But just as the clavicle in the cat only tells of the existence of some earlier creature to which a collar-bone was useful, precedents survive in the law long after the use they once served is at an end and the reason for them has been forgotten.” If some laws seem to be artifacts, Holmes qualifies, they are not likely to burden the people subject to them, for their effect is in their use, and anyway it is only a matter of time before they are overgrown by the “secret root from which the law draws all the juices of life,” which is to say the legislature.
[i] The literature on this subject is enormous. The distinction between legal positivism, natural law, and the common law has been the object of discussion among so many jurists and jurisprudents over centuries that it is impossible to recommend a single text on the topic that would clarify all competing views. The most authoritative voice on matters of positivism today is probably Joseph Raz.