In this article, the author discusses problems that recur in international arbitral awards applying public international law. The controversial example of the concept of ‘abuse of right’ - whether it exists and, if so, whether it may be applied as a rule of decision or it is only a general proposition - is used to illustrate the extent of the problem in deciding cases. The article is the edited text of the Virtual1 Kaplan Lecture delivered online by the author on 9 November 2020, co-sponsored by HKIAC and the British Institute of International and Comparative Law (BIICL).