What Makes an Administrative Decision Unreasonable?

The nature of reasonableness review in administrative law has long been obscured behind vivid but uninformative descriptions. In recent years, courts and commentators have recognised that reasonableness review involves assessment of the weight and balance of reasons bearing on a decision. Yet by itself this idea is substantially incomplete, for there are many ways in which issues of weight might be relevant. Drawing on the theory of practical reason, this article offers a new account of the reasonableness standard that explains precisely how the weight of reasons matters. It shows, negatively, that several existing accounts are mistaken. Positively, it proposes that reasonableness be understood as a requirement of ‘relativised justification’: a decision must be justified relative to some eligible understanding of the balance of reasons. This account explains the standard's central features and yields a coherent, workable test for courts to apply.