Starting from the assumption, that rules of “public international law” are not a priori different from other rules of law, inasmuch as they may directly have legal effects similar to those of, e.g., the rules of municipal law, there are some analogies, at least in technique, between the relationship public international law — municipal law and the rules of conflict of laws. In dealing with the problem of combining international law rules — particularly those laid down in treaties — and rules of the municipal legal system one should be careful to take into account the difference in “environment” of the two groups of rules, in order to avoid either under-, or overrating the effect of each of those groups of rules in situations where both are applicable.

In particular the recognition of international law as “higher” law should not be considered as implying a duty of the municipal courts to apply its rules beyond their actual scope. Examples are given of such overrating in recent commentaries of some authors. On the other hand, a possible tendency to underrate the effect of international law rules is illustrated by examples drawn from a recent judgement of the Netherlands Supreme Court and from the jurisprudence of the German Bundesverfassungsgericht. It is submitted that a conflict may sometimes be avoided (or solved) by adaptation of the application of rules of municipal law in cases where the application of international law is also involved.

The direct application of international law rules is distinguished from its application by national courts in respect of “incidental questions”, i.e. in cases where a rule of municipal law makes a legal relationship, not governed by international law, dependent upon another legal relationship, determined by rules of international law. In such cases municipal law adds to the rules of international law a legal effect, not contemptated by those rules. The interpretation according to which articles 65 and 66 of the Netherlands Constitution would envisage a general “addition” of this kind is discussed and rejected.