In October, I received an email from a student representative of Oxford Stand Up to Racism inviting me to sign a statement headed “The far right are not welcome on our campuses”. This was part of an effort to prevent Alice Weidel of Alternative für Deutschland from giving an invited talk at the Oxford Union. This email arrived two days after I had received an email from students at the American University of Beirut insisting that I withdraw from giving an invited lecture there. One of them, apparently endowed with superhuman resistance to boredom, had searched my CV and been rewarded by discovering that I am an adviser to the Center for Moral and Political Philosophy at the Hebrew University in Jerusalem. I am thus in violation of the academic boycott of Israel and, the students claimed, a paid employee of an institution that is implicated in war crimes. I was, therefore, “not welcome” at their university.

I replied to the first email by explaining why, although I share the students’ detestation of AfD, I believed that preventing Weidel from speaking would be counterproductive. I replied to the second by explaining why I would not withdraw and offering to meet the students for discussion.

In the sequels, Weidel withdrew and I was shouted down for 20 minutes by students with placards and Palestinian flags, after which I was allowed to speak – though not to the protesters, who had obediently filed out on orders from their leader, who was not a student but, the university discovered, a Hezbollah activist. It made no difference that my work for the Hebrew University is both trivial and unpaid, that the philosopher who asked me to do it had been imprisoned for refusing to do military service in the Occupied Territories, and that everything I have published about the Palestinian-Israeli conflict has been critical of Israel, even to the point of arguing that settlers in the Occupied Territories are morally liable to defensive harming because of their collaboration in the theft of Palestinian land.

These events are instances of a broader “no-platforming” phenomenon: a recent surge in efforts to suppress the expression of views that people find offensive or immoral, and to punish those associated with those views. These efforts take a variety of forms. The academic boycott of Israel (an element of the larger movement for Boycott, Divestment, and Sanctions) seeks to exclude and isolate all Israeli academics. In other instances, such as the two I have cited, students mobilise to prevent invited speakers from speaking. There have also been campaigns to ostracise individual academics and force them to retract their published views, as well as efforts to block academic appointments and to have academics removed from their positions because of the substantive content of their writings.

These efforts have come from both ends of the political spectrum. Efforts from the right have included harassment and defamation of climate scientists, death threats to philosophers who have argued for contentious views in bioethics (such as that infanticide can sometimes be permissible), protests over the philosopher Peter Singer’s appointment at Princeton (which included not only death threats to Singer and the president of the university but also threats from conservative benefactors, which were eventually fulfilled, to discontinue donating to the university), and the cancellation by the University of Illinois of its contract with a Palestinian-American professor who had expressed anti-Israeli views on social media. Efforts from the left have included attempts to coerce the retraction of an article on transracialism by philosopher Rebecca Tuvel and the shouting down of a scheduled debate at Middlebury College between Charles Murray, the controversial co-author of The Bell Curve, and Allison Stanger, a liberal political science professor who was attacked by students as she left the building and had to be treated for both a concussion and whiplash.

There are, of course, graver threats to free and impartial inquiry than those posed by the forms of action I have cited. In the West, most are posed by conservative governments, such as the Trump administration, and the distortions of truth their supporters circulate through social media and the internet. Similarly, the most obviously impermissible means of suppressing ideas – threats of physical violence, organised efforts to defame or discredit individual academics, and so on – are employed mainly by people or groups on the right, outside the universities. Yet protests intended to prevent invited lecturers from speaking, public denunciations of academic heretics accompanied by demands for recantation, and other similar tactics come primarily from the left, within the universities.

These latter efforts at silencing academics seem to me often unjust and almost always self-defeating. The boycott of Israeli academics, for example, is unjust because it wrongs Israeli academics who actively oppose their government’s policies of occupation, settlement, and blockade, often at considerable personal cost. These individuals have done nothing to make themselves liable to be boycotted. To shun and seek to isolate them simply because they are citizens of Israel is a form of unjust collective punishment. It is also counterproductive not only because it risks alienating the critical voices within Israel but also because it squanders the opportunity to co-operate with them, when they are in a stronger position to influence their government than those who are not citizens of Israel. Justice for the Palestinians will not be achieved through force but only through mutual understanding and reconciliation, which require discussion that is open and respectful on both sides.

It is not unjust simply to decide not to invite people to speak at one’s university because one objects to their views. No one has a right to be invited to speak at a university and, given that the choice of speakers is necessarily highly selective, it is a reasonable ground for excluding certain people that their views are repellent. But once a speaker has been invited, and especially if this person has arrived for the event, it may be unfair, both to the speaker and to those who have offered the invitation, to use force or intimidation to prevent the event from occurring. It is also usually highly counterproductive. Preventing invitees from speaking by shouting them down gives them and those who agree with them no reason to rethink or abandon their views. Rather, their opponents forgo a rare and valuable opportunity to explain to them, politely but cogently, why their views are mistaken. In the words of St. Bernard’s admonishment of the clergy, “errors are refuted by argument, not by force”. Indeed, silencing speakers rather than engaging with their ideas may suggest to them and others that one lacks confidence in one’s ability to refute those ideas or to defend one’s own.

Those who organise or support protests at university lectures often say that their aim is to deny the speaker the legitimacy conferred by the opportunity to address an audience at their university. But I doubt that there is any such conferral of legitimacy, or even a perception of it. University lectures usually pass unnoticed except by a few within the university. And, in any case, universities and academics no longer command the respect they arguably once did, especially from people on the right, who have become increasingly dismissive of academics, including scientists. Yet even if some form of recognition were conferred, it would derive from the invitation by the institution, not from the actual giving of the talk. A speaker who has been invited but cancels because of illness does not thereby forfeit any recognition conferred by the institution. Nor is that recognition negated if the talk is disrupted by some tiny minority of the student body.

The general effect of forcibly preventing or disrupting a talk is usually quite the opposite of denying legitimacy to the speaker; such speakers gain publicity that they probably would not otherwise have had. The bullying they have experienced hardens their resistance to the views of their opponents and enhances their standing among their supporters, who can now regard them as heroes and martyrs. And those who have hitherto been unaligned with either side are likely to be alienated by what they may perceive among the protesters as intolerance, vindictiveness, and a preference for force and intimidation over reasoned discussion. Although the protesters and their sympathisers may enjoy a strengthened sense of rectitude and solidarity, they are likely overall to have damaged the cause they seek to defend.

Another peril in silencing invited speakers at universities is that it invites reprisals. Suppose conservative students declare that “the far left are not welcome on our campuses” and begin to shout down speakers on the left, such as those advocating the permissibility of abortion. It seems doubtful that students who now shout down conservative speakers will acquiesce in this; rather, they will respond with counter-protests at the same events. Or conservative students might organise counter-protests in defence of the speakers they have invited. The resulting confrontations are likely to escalate into affrays, with mobs of students screaming angrily at one another, potentially erupting into violence. At present, students on the left may outnumber those on the right, but moral, political, and scientific disagreements should not, and indeed cannot, be resolved by the balance of forces.

In international relations, the analogue of addressing disagreements through silencing is the choice of war over diplomacy. War is normally ruled out as impermissible by the requirement of necessity, which demands that one choose, from among the possible means of achieving a just aim, the one that can be expected to have the morally best balance between effectiveness and minimising harm to others. If the disruption of invited lectures is normally not only ineffective but self-defeating, it seldom has any chance of satisfying the requirement of necessity.

Except when speakers overtly incite their audience to violence or hatred, the appropriate way to respond to speakers whose views one believes to be offensive or immoral is to allow them to speak and then, in a temperate manner, challenge their views with reasons, arguments, and evidence. Harassment, bullying, and intimidation are tactics unworthy of universities and of the left. We should leave them to Trump and his minions.

Jeff McMahon is the White’s Professor of Moral Philosophy at Corpus Christi College, Oxford. Jeff is the author of Killing in War and co-founder of The Journal of Controversial Ideas.