University of Illinois Law Review, Vol. 1984, No. 3 (1984), pp. 561-576


A proper understanding of the nature of customary law is important for legal historians. For students of European legal history, customary law is particularly important; from post-Roman times to the beginning of the modern legal age in the eighteenth century, the two main elements in European law have been Roman law and legal custom. In large measure, the main task of lawyers of that interim time period was to unify or harmonize the two strands of Roman law and custom.

Customary law flourishes in circumstances where law is likely to be the least theoretical. Yet, the nature of any source of law requires theoretical underpinnings regardless of whether these underpinnings are always implicit and never expressed. Accordingly, for custom to be regarded as law in Western private law, more than simple usage must be and is required, even if the usage is general and has long flourished. The principle issue is that one cannot derive an ought from an is. Consistent behavior in accordance with particular implicit rules does not indicate that people should so behave, or conversely should be subject to some sanction if they do not.

The main problem for any theory of customary law seems to be determining the nature of the additional factor required to transform custom into law. The Roman sources clearly indicate that some additional factor is needed to recognize custom as law, even if the nature of this factor is not apparent. For example, the Epitome Ulpiani states that "[c]ustom is the tacit consent of the people, deeply rooted through long usage." The additional factor is expressed by the otherwise tautological "tacit consent" or "tacit agreement" (tacitus consensus). But, this approach raises the question as to what has tacit consent been given? Certainly, tacit consent is not given to the long usage itself, although the tacit consent is rooted in the long usage. Another Roman source, the Justiniani Institutiones, states that "[u]nwritten law is that which usage has approved. For long-practiced customs, endorsed by the consent of the users, take on the appearance of statute." In this instance, the additional factor is expressed by "endorsed by the consent of the users" (consenu utentium comprobati).

Nevertheless, the vagueness of Ulpian is not dissipated as a result of this other explanation. The Justiniani Digesta, however, clarifies the nature of the additional factor by stating that

"[d]eeply rooted custom is observed as a statute, not undeservedly; and this is what is called law established by usage. For since statutes themselves bind us for no other reason than because they have been accepted by the judgment of the people, then deservedly those things which the people have approved without writing will bind all. For what does it matter that the people declare its wish by vote or by positive acts and conduct? Therefore, it is very rightly accepted that laws are abrogated not only by the vote of him who proposes law, but also through desuetude, by the tacit consent of all."

For Julian, the nature of the additional factor seems to be clearer: apparently the custom is law because the people accept it as law.

This article discusses the dominant theory which legal historians have adopted to explain how custom is transformed into law: opinio necessitatis. Although legal historians generally accept the doctrine, it has a number of theoretical failings which hamper its usefulness as an explanatory tool. Accordingly, this article analyzes those failings and an alternate theory that custom becomes law only when it is the subject of statute or judicial decision. Finally, the article proposes nine propositions related to the role of judgments in creating customary law.

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