Why write a book on custom? To this day I can still remember one of my first international law classes at the Université de Montréal in 1993 when Professor Jacques-Yvan Morin explained the mysterious way that customary rules emerge. He brilliantly told us that these rules emerge in the same way that footsteps through a field eventually transform into a pathway followed by all. It was only many years later that I discovered that the analogy was actually introduced by Professor Cobbett and later refined by Professor de Visscher:

Ever since these early law school days I have remained fascinated by the phenomenon of the formation of customary rules. Back then, however, it never crossed my mind that some 20 years later I would actually write a book on the topic.

The idea of writing a book on custom emerged from my own experience as a lawyer. Before becoming a professor at the University of Ottawa in 2009, I practised law for about ten years in the field of international arbitration in Geneva (at Lalive and at Lenz & Steahelin), in Montreal (at Ogilvy Renault, now known as Norton Rose Fulbright) and in Ottawa (at Canada's Ministry of Foreign Affairs and International Trade, ‘Trade Law Bureau’). For the first time, in the context of these arbitration proceedings, I had to concretely apply the theoretical concept of customary international law that I had learned in law school to real facts in real cases. This is when I truly discovered the multifaceted complexity of the principle. At the time, I desperately looked for scholarly works examining the application of custom in the specific field of international investment law. No such work existed.