Keele Research Repository

Ever since the Wednesbury decision in 1947 UK public law has been applying the concepts ‘rationality’ and ‘reasonableness’ indistinguishably. Rationality has also been used as a ‘mega ground of judicial review’, covering many other, distinct grounds of review. The main purpose of this paper is to promote conceptual clarity in UK public law by indicating the nature of rationality as a distinct ground of judicial review, explaining why it should not be used as mega ground of review, and highlighting the overlooked differences between reasonableness and rationality. It is argued that (1) reasonableness is in its essence a balancing and weighing test; (2) the most accurate way of understanding rationality review in public law is to perceive it as ‘instrumental rationality’ or as a ‘suitability test’ that reviews the logical and causal connection between means and end; (3) this ‘instrumental’ perception of rationality is already applied in UK public law as part of the proportionality test; (4) rationality, unlike reasonableness, is not a weighing and balancing test, thus it is wrong to use the concepts ‘reasonableness review’ and ‘rationality review indistinguishably’; and (5) it is conceptually wrong and confusing to use rationality as a ‘mega ground of review’.