Finding the golden mean between many different rule sets
One international law scholar has observed that, "The determination of customary international law is more an art than it is a scientific method."
Determining customary international law is esoteric and amorphous. Since there is no one body of rules for governance, no one criteria or standard for beginning (or ending) conflict, nor is there a bellwether for conduct between states, it is very difficult to strictly codify the "rules of international law". Approaches such as "traditional positivism" have tried to address the issue, looking at general principles of law held in common by states through practice. (Ku, Diehl 25-29) However, as critics have charged, "positivism tends to place too much emphasis on the absolute sanctity of treaties" (Ibid. 29).
Judges in cases of international consequence must look at many varied sources in order to find the right solution. They cannot rely on one codified manual or text in order to find the relevant case(s) for each instance. They must look at the laws of the respective countries, common law practices, precedences, history, culture, etc., in order to find a balanced, appropiate, and above all, fair ruling. Due to the esoteric nature of international laws in the system today, there is a need for open-mindedness and artistry.
Source: Ku, Charlotte, and Diehl, Paul F. International Law: Classic and Contemporary Readings, 2nd ed. Boulder, CO: Lynne Reinner Publishers, 2003.