Every spring a pro-life group – one whose campaigning methods are so shockingly offensive that I won’t publish their name here – sets up shop on my university’s campus quad. The group’s shtick involves displaying billboard-sized images of aborted foetuses juxtaposed with gory photos of atrocities such as mass graves and lynchings.

The group has been haunting me for years; when I was in grad school, their designated free-speech zone happened to be right outside my cubicle window. Most years, I took advantage of my location to plaster the window with pro-choice signs of my own. A few years ago, they visited the campus where I’m now an assistant professor, setting up their grisly billboards in a 20-foot circle right in the middle of campus. Several of my students, tickled at the prospect of witnessing their visibly pregnant, feminist ethics professor debate the morality of abortion, managed to convince me to try to talk to the protesters. It went about as well as you might expect.

What went considerably better was the response from the rest of the student body. After the initial shock wore off and they came to understand that, as a public university, we were obliged to respect the protesters’ rights to free speech, the students mounted a spirited counter-protest. A coalition of many different student groups got involved – not just the usual suspects from the pro‑choice camp (sex-positive feminist types, someone dressed up like a seven-foot Gumby doll handing out condoms, etc) but also, surprisingly, many who identified as pro-life.

Some of the pro-life students were Cambodian (our city has a large and vibrant Cambodian expat community), and they rightly took umbrage at the group using photos of the Cambodian killing fields for their crass political purposes. Others were African American or Jewish, and similarly incensed at the way the group had appropriated images of lynchings and the Holocaust.

The pro-life group’s central strategy is to invoke emotional reactions of disgust in the viewers of their billboards. We see a gory photo of a mass grave and our immediate response is visceral. This tells us something about the moral status of the situation, about the immorality of this horrifying lack of respect for the value of human life. Then we see a gory photo of an aborted foetus and, again, we have a visceral response of disgust. The group’s strategists want us to think that we’re having the same visceral response in reaction to both photos. And they want us to conclude that the moral status of both situations, mass graves and abortions, is the same.

The problem with this strategy is that our emotional reactions can mislead us. I’m squeamish about a lot of things – not just lynchings and abortions, but also bugs, the thought of bats getting caught in my hair, and blood and guts in general. And these different things don’t share the same moral status. Whenever I stumble across one of those sciencey TV programmes that ‘contain graphic content depicting medical procedures which, when viewed by those under 15 years, may require the guidance of an adult’, my nervous system will insist that I’m witnessing an act of utter atrocity, something so viscerally horrific that if I don’t flip the channel immediately I’ll either vomit or pass out. So I change the channel. But I know my viscera are misleading me. I know full well that there’s nothing wrong with open-heart surgery. The question is, when should I listen to my guts and when should I ignore them? What makes disgust morally relevant in some situations but not others?

Historically, the question about the role of the emotions in moral reasoning has resulted in two philosophical camps: the Humeans and everyone else. In his Treatise on Human Nature (1739), David Hume argued that reason, by itself, can tell us nothing about morality: ‘’Tis not contrary to reason to prefer the destruction of the whole world to the scratching of my finger,’ he notoriously insisted. Since it’s our emotions that provide the motivation for our actions, and since moral judgments are uniquely capable of motivating us to act or refrain from acting, this means that morality must include an ineliminable emotional element. Contemporary Humeans call themselves ‘noncognitivists’, to indicate their view that morality has no cognitive content, that strictly speaking there’s nothing true or untrue about it.

You might think the disgusted pro-lifers would find support for their tactics in noncognitivist ideas such as these. Didn’t Hume teach that the way you feel has ultimate authority in moral questions? Contemporary Humeans also emphasise that one of the primary reasons we engage in moral discourse is to influence people’s behaviour. As anyone who’s worked in advertising or watched a few seasons of the US TV series Mad Men knows, appealing to emotional considerations can be an effective way to get people to do what you want. Obviously, these anti-abortion protesters are trying to influence the students on my campus; there’d be no reason to go to such lengths if they weren’t.

But even the most apparently permissive Humean might have a few things to say when confronted by those abortion posters. For starters, we could ask the protesters why this disgust is so overwhelming, when they’re perfectly happy to change their baby’s diapers or play with their pet tarantula or whatever. In my experience, the process of answering this question often reveals hidden confusions. When I press them to articulate why the repulsiveness of an aborted foetus is equivalent to that of the mass grave and not, say, the cardiac unit, more often than not I’m greeted with confused silence. ‘It just is,’ they blink. But that’s not going to convince anyone who doesn’t already agree with them.

The point of moral discourse isn’t to persuade your opponent by any means necessary; it’s to convince them with reasons

Furthermore, grossing people out with gory pictures might be an effective way to discourage some people from seeking abortions, but it obviously doesn’t work on everyone. And if you happen to not be disgusted by their posters, or if your countervailing motives are stronger than your revulsion, then they’ve exhausted their arsenal.

What these protesters need are reasons. And not every reason counts as a good one. Ever since Socrates derided the sophists who held sway in his ancient Athens, philosophers have been taken with the idea that respecting people requires engaging with them as rational equals, not manipulating them with rhetoric. The point of moral discourse isn’t to persuade your opponent by any means necessary; it’s to convince them with reasons. Otherwise, ‘making you an offer you can’t refuse’ would be a totally legitimate way to get someone to change her mind.

In effect, we have turned Hume on his head: morality actually includes an ineliminable rational element. When our emotions are morally relevant, they are backed up by reasons and arguments. And so, whether or not that offensive group is right about the morality of abortion, once the sensationalism of their tactics has grabbed our attention, what they owe us is a convincing argument about why abortion is the moral equivalent of the other atrocities they depict.

And what happens then? Well, here’s a pretty standard pro-life argument. Suppose our protesters declare that a foetus is a person from the moment of conception, and that performing an abortion is therefore the equivalent of murder. ‘Abortion stops a beating heart,’ as the slogan goes.

That, at least, gives us something to work with. As it happens, I don’t believe that a mere clump of cells is sufficient for personhood. If you ask me why abortion is morally permissible, I’ll start with the fact that a foetus doesn’t have a developed capacity to set and pursue ends according to reason, which is what persons have that makes it wrong to kill them. Then I’ll point out that, at most, what a foetus has is the potential to have these rational capacities. I’ll argue that an actual person’s right to control what happens to her body trumps whatever rights a potential person might have. For these reasons, I’ll tell you, abortion is morally permissible.

Of course, I’m sure the more reflective pro-lifers take themselves to have responses to my arguments, and I take myself to have responses to them, and so on. At this point, those who lack the stomach to natter back and forth ad infinitum (which is to say, those who have better things to do because they aren’t philosophers) might ask when exactly this exchange of reasons is supposed to stop. In uncontroversial moral truths? In appeals to some particular aspirations shared by as many of us as possible? Or do they carry on forever?

A Humean would probably appeal to the personal motivations of her audience, trying to get them to see how the desired conclusion follows from some goal or value that they already hold dear. This strategy has the advantage of restoring moral significance to the emotions in a way that meshes with many people’s pretheoretical intuitions. It also sits neatly with the experimental evidence of social intuitionists, who claim that most of our moral judgments occur below the threshold of awareness and that reason plays at most a post hoc role in justifying them.

Still, one might feel a little uneasy at the optimism of this approach. It assumes that our opponents actually have fundamental values that would allow us to derive our conclusions. But an opponent who happens not to care about the right things will have no reason to agree with us. If the firmest bedrock we can hope for in ethical debate is a set of contingently shared inclinations, we are in a precarious position.

What we really want is a universal standard, something we can appeal to that shows our opponents why they should have certain fundamental convictions. And so, unfashionable as it might be, I’m holding out hope for foundational moral truths. But whichever road we take, notice how far away the claims of visceral disgust now seem. At this point, we’re firmly in the game of exchanging reasons. Disgust and other morally loaded emotions might have a role to play in this game, but it’s ever only the beginning of a conversation, not the end of it.