International courts and tribunals have almost unequivocally expressed their resolve to follow their precedent unless they find very particular reasons to do so. In this connection, it is recognized that Article 38 of the ICJ Statute generally contains the sources of international law. Among these sources are "judicial decisions" but only in a non-binding fashion –i.e. subject to Article 59 of the ICJ Statute– and as a "supplementary means for the determination of rules of law". Legal scholarship has struggled to reconcile the subsidiary and non-binding place of precedent in the pyramid of the sources of international law under Article 38 of the ICJ Statute with the treatment that is in practice accorded to it in international adjudication. This confusion has led even the most prominent commentators to conclude that international courts and tribunals effectively view precedent as a "quasi-formal" source of international law, "de facto" mandatory precedent, "quasi-precedent", or a delegation of "judicial legislation".

Against this background, this book seeks to ascertain the reason why international courts and tribunals have recognized their resolve to adhere to their precedent. It finds, as a preliminary matter, that the long-standing debate about whether precedent is legally binding fails to advance the discussion. International courts and tribunals rely on precedent not because they find themselves bound to do so. Rather, precedent in international adjudication is a form of legal reasoning whereby a solution to a current legal question is found by ascertaining the solution given to an earlier similar question. Reasoning from precedent, as an independent form of legal reasoning, consists of two discrete elements. First, the adjudicator finds an "example" that shares relevant similarities with the case at hand. These similarities may be perceptual, analogical, or a mix of both. Second, the reasoner assesses the "authority" of that example. This authority should provide the necessary legitimacy or weight for the example to be relied upon in the subsequent case.

When an example has sufficient authority, there is no reason for the adjudicator not to follow its precedent. This explains why international courts and tribunals reason from precedent as a matter of course, absent cause or reason not to do so. At the same time, this approach to precedent is sufficiently flexible to allow international courts and tribunals to depart from, or amend, any precedential rules when there is cause or reason to do it.