The facets I'm thinking of are:
- The main thing people who are pro-life or pro-choice seem to disagree on is the status of the foetus. One group says it's a baby, the other says it's a piece of tissue. That difference is crucial, as it determines whether an abortion is murder or the removal of unwanted tissue. Something I think is often missed in 'debates' about abortion is that no one actually thinks killing babies is OK or that removing unwanted tissue is evil. People on different sides of this issue disagree about what is going on, not about ethics per se.
- Very few people that I've encountered (either in person or online) actually think the foetus is either a baby or a piece of tissue. This comes out in the ways they speak and act. Pro-choice people often talk about abortion being a difficult choice (like is being done here, for example) in a way that they would never talk about an appendectomy. Pro-life people rarely obstruct abortions with the same dedication people have shown at school shootings. Those aren't perfect comparators (your infected appendix is likely to kill you in a way that your foetus isn't; abortion is state-sanctioned and ongoing in a way that school shootings are not), but I still think they're revealing. I think they show that, in reality, the abortion 'debate' is much less polarised than we think. The majority of the population seem to think that the foetus is some kind of 'proto-human', even if some individuals tend more towards the 'proto' and others toward the 'human'.
- Science doesn't have anything to say on the status of the foetus. It's just not the kind of question science can answer. It is, of course, living human tissue, but so is your appendix. It can, of course, develop into a human given the right conditions, but so can an ovum or even a skin cell, depending on how broadly you wish to define 'the right conditions'. It isn't, of course, able to survive on its own, but neither can a new-born baby or even a toddler. Although people on both sides of the 'debate' claim that science is on their side, this question of the status of the foetus is something we need to figure out philosophically. Science just doesn't deal with this kind of question. (And, as my previous posts showed, I don't think the Bible is very helpful with this either: it's pretty clear on the importance of preserving life, but has little or nothing to say on where it starts.)