TragicMonkey said:
Really, the whole abortion question is rooted in the problem of determining what is life, and what is man? Which seem to be two of the fundamental questions that the entirety of philosophy seeks to answer.
Those are both actually red herrings, sorry. Neither "What is life?" nor "What is man/woman/human" is the pertinent question.
The pertinent question is, "What is it about adult, normal humans that makes their lives special?".
I'm assuming, of course, that our goal is to have a moral view with a basis in observable facts rather than one pulled out of thin air. If normal adults are special and morally valuable, so that we shouldn't kill them, what precise qualities are we basing that judgement on?
As far as I'm concerned, the answer is that adult humans are special because of their complex mental lives. I am also fairly sure most people agree with this if you pin them down on it, since few people have a problem with turning off the life support for human bodies whose mental lives have irrevocably ceased. Extending this view to human bodies whose special mental lives have not yet begun is very hard to avoid, but nonetheless people will do their damnedest to avoid making the connection.
It's not going to be easy to arrive at a solution, and that's why I distrust the easy-sounding answers on both sides of the debate: I suspect neither side has sat down and thoughtfully considered all the ramifications, explored their definitions and what they necessarily hinge upon, or plumbed the depths of the concepts involved. It seems to boil down to "God" versus "my body, my business", which seems way too simplistic.
But then, I have a tendency to overthink things.
I think that's a bad summary, and that if you've overthunk some aspects of this issue you've also underthunk some of them.
The reason this issue is a problem is because our culture has a terribly strong taboo against infanticide. We think that our taboo against it makes us morally superior to other cultures that practised infanticide or still practise it. The fact that I've said this at all will probably pull some well-meaning sort like Harry Keogh out of the woodwork who will feel absolutely compelled to explain that anyone who questions this taboo is a sick, sick subhuman.
Meanwhile (as a society) we are happy to unplug adult "vegetables", and we constantly kill cats, dogs, sheep, cows, chickens and horses because they are unwanted or tasty, despite the fact that by any objective yardstick these creatures are more deserving of respect and consideration than a six month old human poop tube.
So while we can manage (as a society) to be sensible about abortion, since out of sight is out of mind, as soon as the fetus looks like a baby the infanticide taboo kicks in.
Not that I'm actually advocating infanticide for healthy infants: the little poop tubes are in enormous demand for adoption, and it's never a problem to find people who will be enormously happy to take charge of an infant that the mother doesn't want to take care of. I'm just saying that if no one wanted one, killing it would be less morally troublesome than euthanasing an unwanted dog. Which is once again not to say that I'm in favour of it, just that our society tolerates it.