Whose reason is reasonable? Reasonableness, negligence, and the mentally ill defendant

[Extract] "Reasonableness" underpins Lord Atkin’s famous judgment, but whose reason it is based on remains unclear. Subsequent judgments have assumed that the conduct of all defendants should be judged against an objective "reasonable person" standard, but "reason" is a presumed and ill- defined characteristic. Lord Atkin’s actual words place him in the defendant’s shoes, suggesting that he may have favoured a subjective test. That possibility has received little judicial consideration.

"Reason" is critical to the insanity defence, which is recognised in criminal law but not tort. For mentally ill or cognitively impaired defendants, whose reasoning capacity differs from that attributed to the "reasonable person", the question of whose reason is of great significance.

Did Lord Atkin intend the test to be subjective? Did he have mentally ill defendants in mind when he formulated the "neighbour principle"? If so, did he intend that negligence law should break with tradition, and recognise a defence of impaired reason? This paper explores whether the current state of negligence law regarding mentally ill defendants deviates from Lord Atkin’s original concept of negligence law, and whether it is time to review negligence law relating to the mentally ill as a particular class of defendants.