“Harassment“ is, according to Lord Sumption (with whom Lord Neuberger and Lord Wilson agreed), “an ordinary English word with a well understood meaning”, which is, he continues, “a persistent and deliberate course of unreasonable and oppressive conduct, targeted at another person, which is calculated to and does cause that person alarm, fear or distress” (following Lord Phillips of Worth Matravers MR in Thomas v News Group Newspapers Ltd  EMLR 78, para 30). Under the Protection from Harassment Act 1997 harassment is both a criminal offence (section 2) and a civil wrong (section 3), but a defendant will avoid liability if he or she can show that he or she pursued his or her course of conduct “for the purpose of preventing or detecting crime”. In Hayes v. Willoughby  UKSC 17 the Supreme Court had to determine whether this meant that a defendant who “sincerely believed” that the claimant had committed a variety of offences, and pursued a campaign that was “subjectively directed at the prevention or detection of crime”, could rely on this defence even if his persistence in that campaign, after being assured that his allegations had been investigated and found to be unsubstantiated, “exceeded even the widest limits of reasonableness and became unreasonable and obsessive”. (The quotations in the previous sentence are from the trial judge’s findings of fact, as quoted by Lord Sumption.)
The Court of Appeal had decided that: (1) It was possible to distinguish between the purpose of a course of conduct and the purpose of the person pursuing that conduct, and if the conduct itself was not rationally connected to the goal of preventing or detecting crime then the person pursuing it could not rely on the defence; and (2) the person could only rely on the defence if his sole purpose was to pursue or detect crime (and as a result a tinge of vengefulness could negate the defence). The Supreme Court rejected both of these propositions. Thus, on (1): “in the context of section 1(3)(a) purpose is a subjective state of mind” (at para , see also para ), and on (2): “the ordinary principle is that the relevant purpose is the dominant one” (at para , see also para ). A majority of the Court, however, (with Lord Reed dissenting), held that a person will only be held to have a particular “purpose” (a subjective state of mind) if he or she has engaged in the “minimum mental processes necessary to acquire the relevant state of mind”, that is “thought rationally”.
A lengthy passage from para  is worth quoting since it relies on a difficult distinction between whether a person has gone through particular mental processes before reaching a particular “state of mind” and whether some notional, reasonable person might have been able to reach the same “state of mind”. Here’s the passage:
“Before an alleged harasser can be said to have had the purpose of preventing or detecting crime, he must have sufficiently applied his mind to the matter. He must have thought rationally about the material suggesting the possibility of criminality and formed the view that the conduct said to constitute harassment was appropriate for the purpose of preventing or detecting it. If he has done these things, then he has the relevant purpose. The court will not test his conclusions by reference to the view which a hypothetical reasonable man in his position would have formed. If, on the other hand, he has not engaged in these minimum mental processes necessary to acquire the relevant state of mind, but proceeds anyway on the footing that he is acting to prevent or detect crime, then he acts irrationally. In that case … the law will not regard him as having had the relevant purpose at all. He has simply not taken the necessary steps to form one.”
In the previous paragraph (para ) the suggestion is made that “thinking rationally about the material” (underlining added) involves applying “a minimum objective standard to the relevant person’s mental processes. It imports a requirement of good faith, a requirement that there should be some logical connection between the evidence and the ostensible reasons for the decision, and (which will usually amount to the same thing) an absence of arbitrariness, of capriciousness or of reasoning so outrageous in its defiance of logic as to be perverse.” Thus a person will only have “thought rationally” if he or she has thought in a way that avoids these failings, and unless he or she has “thought rationally” the law will not be able to conclude the he or she had the requisite “purpose”.
Lord Reed dissented. He argued that additional limits should not be read into statutory provisions, in particular where their effect will be to extend the scope of a serious criminal offence. Moreover, he expressed doubt about whether juries, or indeed judges, would be capable of understanding the distinction and applying such a “rationality” test to the facts. He also suggested that Parliament might have concluded that a simpler subjective defence – perhaps (my formulation), “did the defendant sincerely understand himself to be acting (predominantly) in pursuit of the purpose of preventing or detecting crime?” – would be appropriate in order to avoid investigative journalists and others being “chilled” by the threat of having to justify their actions.
The majority’s answer to the first and third concerns would, presumably, be that their decision simply illuminated the limits that Parliament actually intended all along. Opinions on those issues are likely to divide in accordance with different opinions about the evidence to be found in the text of the statute and about how the task of “statutory interpretation” ought ideally to be conducted. But what of the criticism that the majority’s “rationality demand” is too subtle for juries (and perhaps also for judges, practising lawyers and legal academics)? On that issue, my personal view is that juries will find it easier to decide whether any particular defendant’s reasoning process was “wholly irrational” than judges have found it to catalogue the ways in which reasoning can be “irrational”.