The term hypothetical means facts that are assumed rather than actual. The term a hypothetical means a scenario composed of such facts. Reasoning from hypotheticals takes two forms. In one form, a hypothetical is employed to view a case in a broader perspective to help decide the case. Hypotheticals in this form have three characteristics. First, the hypothetical differs from the case in its particulars, but as a matter of social morality, social policy, and experience the hypothetical is comparable to the case in its basic structure. Second, intuitively the hypothetical is easier to decide than the case. Third, because the hypothetical and the case are comparable as a matter of social propositions, the hypothetical and the case cannot be justifiably distinguished. That being so, the result in the case should be the same as the result in the hypothetical. In the second form of reasoning from hypotheticals, a hypothetical is employed to show that a case should not be decided in a certain way because if the case were decided that way, hypothetical similar cases would have to be decided the same way and deciding those cases the same way would be unsound.

One characteristic interpretive technique in the discourse of customary international law is the identification of such norms as 'possibly emerging' or possibly in existence. Thus it is frequently asserted that a putative norm 'may' have or 'probably has' customary status. This hypothetical mode of analysis can give rise to the speculative construction of international obligations driven more by preference than by evidence. This speculative rhetorical technique is examined by reference to the account of temporal dimensions of the emergence of customary international law provided in the Chagos Archipelago Advisory Opinion of 2019. Here the International Court of Justice endeavoured to pin down the time of origin and path of evolution of a customary norm requiring territorial integrity in the context of decolonialisation as self-determination. This chapter engages with this ubiquitous characteristic of the interpretation of customary international law and argues that the accompanying opacity in relation to international legal norms – norms that are held to generate obligations – is to be deplored.