Two anti-abortion activists were tried for a public order offence for publicly displaying a large, gruesome poster of an aborted fetus of 21 weeks gestation. Some passers-by felt offended by the picture, which the activists had displayed while campaigning to be elected to the Welsh Assembly for the Pro-Life Alliance. The police rolled up the picture and arrested the campaigners. During the court case, the prosecution claimed that the poster was insulting and harassing, while the defence said that all they had done was mount a poster.

No doubt most people who feel 'insulted' or 'harassed' by such images are opposed to the anti-abortion lobby, and support a 'woman's right to choose' abortion. Indeed, it is a fair guess that many who are against showing images of dead, mutilated human fetuses have no objection to showing pictures of mangled foxes torn apart by hounds - at least, as anti-hunt propaganda rather than as trophies on hunting folks' mantelpieces. For such people, the gruesome nature of the material is not the issue - what matters is the moral view it implicitly conveys. Is it possible, though, to disagree with the anti-abortion lobby while being uneasy about the impulse to censor their publicity materials?

It certainly should be, and it is a shame that so many opponents of the pro-lifers want such material banned. But apart from issues of taste, which merit separate consideration, what are the best arguments for keeping such images from public view? To say that the picture insults and harasses is odd. It is unclear in what way such pictures are insulting - unless being exposed to views one disagrees with constitutes an insult. And if talk of harassment is supposed to refer to causing distress (actually it doesn't quite mean that, but let that pass) then the obvious question is why the material should cause distress. Could causing distress be legitimate? And even if not, is that a reason for legally suppressing the material?

Of course, one reason for being distressed, at least for the squeamish, is that the picture might seem aesthetically disgusting. Moreover, even pro-life sympathisers might find themselves upset by it, since it graphically reminds them of a procedure they already regard as murderous. But that surely isn't all there is to it. For other people, some of whom will have had abortions themselves, must be thinking: 'I support the right to choose, and these horrible campaigners are trying to make me feel shocked or guilty, or insinuate that I support murder, and they shouldn't be allowed to do that!'

In other words, they feel got at. But the important question is whether they should feel shocked or guilty; it is simply question-begging to condemn the display just because its (alleged) purpose is to produce these reactions. If the pro-life people are right, then horror of abortion is the only proper response to their campaigning materials. Of course, there is also the aesthetic issue, which I leave on one side; even a convinced pro-lifer might object on grounds of taste or (in a sense) decency, just as opponents of the death penalty might be against showing graphic footage of actual executions.

But the central objection, I suspect, is that the display is intended to shock in the furtherance of a cause they object to. But then, how can this justify preventing the stuff from being shown? Why should disapproval of a cause justify preventing it from being defended in the way its supporters see fit?

It would be different if those opposed to such images could show that their use was dishonest or misleading. And, in fact, this claim is sometimes made, in that lurid pictures of late-aborted fetuses give the impression that abortions are typically carried out late in the second trimester or even in the third, when in fact the majority is performed in the first.

One might add that if they were more honest, the pro-lifers would display pictures of fetuses aborted after six weeks gestation - the reason they don't being that they would have much less shock value. There is something to this, but the pro-lifers have an answer: second-trimester, and in rare circumstances third-trimester abortions, can be legal in the UK. And such abortions can indeed entail the mutilation of the fetus. So how are the pictures misleading, concerning those abortions?

If we are to challenge the pro-lifers in a sensible way, a different strategy is needed; one that has been obscured by rhetoric and irrelevant protest on both sides. How, we should ask, do lurid pictures contribute to the ethical case against abortion? Even for those who consider abortion to be the ending of a life, the question is not whether abortion is killing but whether it is murder, which means wrongful killing. This is not proven by lurid pictures, any more than photos of mutilated foxes clinch the case against foxhunting.

But images can have tremendous power, influencing judgement for strictly irrelevant reasons. The blood and dismemberment assume a disproportionate significance, in comparison to genuine moral concerns about causing pain and/or depriving an individual of a future. Such psychological reactions are easily exploited by image-makers, even in good causes.

If pictures of bloody, dismembered fetuses do mislead, then, it is for this subtle reason, not because they strictly misrepresent any facts. But even so, that seems a weak case for banning the material - no better than the fact that such images upset people. It is much better to say: yes, these pictures represent something that many believe is morally repellent. So let's rationally discuss whether it really is wrong. And then we are led back to the seemingly interminable debate about abortion. But that's another story.

Dr Piers Benn is lecturer in medical ethics at Imperial College London

This article was first published on www.spiked-online.com