(b)
international custom, as evidence of a general practice accepted as law;
(c) the general principles of law recognized by civilized nations;
(d) subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.
Statute of the International Court of Justice, art. 38, P 1. According to Ian Brownlie:
The first question which arises is whether paragraph 1 creates a hierarchy of sources. They are not stated to represent a hierarchy, but the draftsmen intended to give an order and in one draft the word “successively” appeared. In practice the Court may be expected to observe the order in which they appear: (a) and (b) are obviously the important sources, and the priority of (a) is explicable by the fact that this refers to a source of mutual obligations of the parties. Source (a) is thus not primarily a source of rules of general application, although treaties may provide evidence of the formation of custom.
Ian Brownlie, Principles of Public International Law 3-4 (4th ed. 1990) (footnote omitted). However, Brownlie goes on to add that:
In general Article 38 does not rest upon a distinction between formal and material sources, and a system of priority of application depends simply on the order (a) to (d), and the reference to subsidiary means. Moreover, it is probably unwise to think in terms of hierarchy dictated by the order (a) to (d) in all cases. Source (a) relates to
obligations in any case; and presumably a treaty contrary to a custom or to a general principle part of the
jus cogens would be void or voidable. Again, the interpretation of a treaty may involve resort to general principles of law or of international law. A treaty may be displaced or
amended by a subsequent custom, where such effects are recognized by the subsequent conduct of the parties.
Id. at 4 (footnotes omitted). The Vienna Convention is relatively new, however, so little such displacement or amendment seems likely to have occurred.