means that this guideline must be, by and large, effective because in the absence of
effectiveness, it is hard to speak of a customary guideline for behavior. This first
demand for the existence of customary law is usually called the demand ofusus,
which is the Latin word for “custom.”
The second condition is that the custom is accepted as a binding one or—and this
is meant to express the same thing—as a legal one.
Earlier we discussed the custom of a set of teenagers to visit the cinema every week. If these
teenagers do not feel obligated to go to the cinema and only see it as a good custom, there is
no customary law. However, the duty to go to the cinema every week would be based on
customary law if they accept their custom as binding and see violations as a ground for
enforcement by collective means (which is highly unlikely).Opinio Iuris The demand that a custom is accepted as (legally) binding is usually
referred to as the demand that there exists anopinio iuris, the view that the custom
is law, or anopinio necessitatis, the view that the custom is binding and necessary.
Customary law cannot be a validity source of law because something can only be
customarylawif it is already considered to be law. In other words, customary law
exists as law in the sense of social rules; custom is merely a source of origin.
2.7 Legal Doctrine and Reason
Custom is a source of origin for the law because the fact that a rule is broadly
accepted as a legal rule suffices for this rule to be a legal rule. There are also other
factors that can bring about that a rule is accepted broadly as a rule of law, and two
prominent examples are legal doctrine and reason.
Legal Doctrine Legal doctrine, the writings of legal scholars in which they
describe the existing law and mix this description with evaluation and proposals
for how the law should be, does not make the described and proposed rules into
rules of law. However, these writings do influence legal decision makers, especially
in cases where the positive law is not clear and where the proposed rules are well
argued. As a consequence, rules proposed in legal doctrine may end up as rules that
exist through being accepted as legal rules.
Reason If rules arereasonable, the chances are bigger that they will actually be
used by legal subjects to govern their mutual relations and will develop into
customary law. If a judge in a common law system must decide whether the present
case is similar to a precedent, and if so to which precedent, he must judge which
similarities and differences are relevant from a legal point of view. Such judgments
are based on reason, and the outcome will become law through thestare decisis
mechanism.
As these examples illustrate, being reasonable is a basis for rules to become part
of the law. This is not because reason is a source of validity but because being
reasonable is a factor that leads to law via acceptance.
2 Sources of Law 35