As a standard of review, ‘objective reasonableness’ has been in the academic spotlight after the Whaling in the Antarctic judgment of the International Court of Justice (ICJ or the Court). The Court's approach was conceptually innovative and seemed to have operated a partial reversal of the burden of proof in favour of the applicant. In response to certain criticisms addressed to that decision, this article makes two claims. First, ‘reasonableness’, while being inherently vague, gives a justifiable degree of discretion to judges, thereby enabling them to make difficult adjudicatory choices without departing from the applicable legal framework. Second, the term finds sufficient support in the Court's case law dealing with state discretion in the implementation of treaties. Both claims relate to the very same core idea: that even if one remains sceptical as to the capacity of the term to enhance certainty, ‘reasonableness’ is a basic conceptual tool that facilitates judicial review in complex cases, including those of a scientific nature.