"for instance, a person entrusted with a discretion must, so to speak, direct himself properly in law […] If he does not obey those rules he may truly be said, and often is said, to be acting "unreasonably"", as meaning that an error of law is a form of irrationality. It is a reference to the exercise of a discretion within the bounds of reason and that a failure to do so may render it subject to challenge in law. As was made clear in the final paragraph of Wednesbury at 233 – 234, the focus is on the irrationality of or reasonableness of its decision "a conclusion so unreasonable that no reasonable authority could ever have come to it"
"32. A more nuanced approach in modern public law is to test the decision-maker's ultimate conclusion against the evidence before it and to ask whether the conclusion can (with due deference and with regard to the panel's expertise) be safely justified on the basis of that evidence, particularly in a context where anxious scrutiny needs to be applied. 33. I emphasise that this approach is simply another way of applying Lord Greene MR's famous dictum in Wednesbury (at 230: "no reasonable body could have come to [the decision]") but it is preferable in my view to approach the test in more practical and structured terms on the following lines: does the conclusion follow from the evidence or is there an unexplained evidential gap or leap in reasoning which fails to justify the conclusion? 34.This may in certain respects also be seen as an aspect of the duty to give reasons which engage with the evidence before the decision-maker. An unreasonable decision is also often a decision which fails to provide reasons justifying the conclusion. 35. I should also emphasise that under the modern context-specific approach to rationality and reasons challenges, the area with which I am concerned (detention and liberty) requires me to adopt an anxious scrutiny of the Decision: see Judicial Review (Sixth Edition), Supperstone, Goudie and Walker at para.8.12. 36. Applying the above approach, I consider that both a rationality and reasons challenge succeed in this case."