Abortion is a horribly complicated moral issue; I used to argue about it a lot and now I don’t: partly because the result tends to be more rancorous than I like in a debate, partly because people’s opinions are far more set in stone than with almost any other issue, but mostly because there’s very little logical basis to hold -any- debating position.

This isn’t entirely true. There is a position that, although I find it objectionable, is internally coherent: there’s an obvious difference between a fertilized and an unfertilized ovum. This is the only clear “is / is not” step in the foetal development process.

After that, there’s an Old Testament-derived belief that the soul appears 14 days after conception (which has driven the UK legislation that embryos created for scientific research must be destroyed within 14 days), but this is fairly hard to justify empirically. Going onwards, the only remaining “is/is not” event is birth.

As Aussie philosopher Peter Singer pointed out some time ago, the only physical difference between an unborn and a born child is its geographical location. BMA ethical committee adviser John Harris has clearly been reading his Singer, making the same point to a House of Commons committee.

Professor Harris’ point has stoked outrage amongst antiabortion types. Which is odd, given that they claim to believe exactly the same thing: that there is no moral difference between killing a foetus and killing a baby. They ought to be pleased that a prominent government adviser agrees with their terminology.

The outrage that greeted Professor Harris’ remarks highlights something that makes the abortion debate particularly fruitless: on an emotional level, very few people (including the pro-lifers and the philosophers) seriously believe that killing a foetus and killing a born baby are morally identical. Yet at the same time, very few people seriously believe that destroying a fertilized ovum and killing a 26-week foetus are morally identical.

Since the main point of a moral system is coherently to codify our intuitive beliefs (maybe I’m being overly positivist here, but this is broadly the conclusion that I drew from studying ethics), any attempt to base your moral take on abortion on either of the clear cut-off points is comprehensively fucked. The only option is to say “err, it’s some point in between” – and this point is necessarily arbitrary.

This is why arguments against total pro-lifers are fruitless: despite being demonstrably wrong, they’ve got the logical high ground. I guess one possible arguing tactic is to use the comparable example of the age of consent… “there’s no physical difference between a 15 year old and a 16 year old, so having an arbitrary age of consent is stupid and wrong. Sexual activity should be allowed at the clear biological cut-off point: the age of puberty” isn’t an argument with great intuitive moral appeal, despite having some logical merit.