Hersch and Elihu Lauterpacht and the relevance of private law sources for international law: from theory to practice against the background of international law's evolution and the adoption of the Unidroit Principles

Abstract Private law sources in international law were a main object of study of Sir Hersch Lauterpacht, who is considered one of the greatest international lawyers of the last century. In his time, the International Institute for the Unification of Private Law’s Principles of International Commercial Contracts (PICC) did not exist. The first version of the PICC was only published in 1994, more than three decades after his death. In the year 2000, the disputing parties of an investment treaty arbitration case concluded before a tribunal chaired by his son, Sir Elihu Lauterpacht, a settlement agreement dated 20 March 2000, which would later be embodied in an International Centre for Settlement of Investment Disputes Additional Facility arbitral award issued by the tribunal on 18 September 2000, where they incorporated slightly modified PICC provisions. The present article will argue that private law sources are still important in certain areas of international law, such as investment arbitration. In investment arbitration, they are not only important in presence of contracts but they can also arguably be useful for the interpretation of treaties. Investment tribunals have indeed recognized the importance of the PICC for both the interpretation of contracts (transnational agreements) and the interpretation of treaties (international agreements).