Well, in this context, I mean a shared understanding of the metaphysical nature of the subject. (Generalising a bit,) People who hold an idealist conception of the subject as "a life", an immaterial "vital spark" prior to concious being, will overwhelmingly be pro-life, because they view a newly-fertilised egg as possessing the same vital spark as you or I. People who hold a materialist conception of the subject as a sentient organism will tend to be pro-choice, because they do not regard a foetus as attaining moral significance until it develops a sentient mind. People who hold a dualist conception of the subject, as immaterial but necessarily sentient, and tend to be moderate on the issue, because they regard the foetus as not possessing intrinsic moral value, but developing it quite quickly, generally at the first signs of neural activity. (Not all of these frameworks are made explicit, not least the number of self-proclaimed "materialists" working in a de facto dualist framework, but I think that it's more or less fair as a rough characterisation of the three main tendencies.) What this means is that we're working with a number of different assumptions about what a person actually is, and thus how it is that we are to act towards other people in an ethical manner; while I might consider a foetus to be nothing more than a blob of cells, a pro-lifer considers a full-blown person (and, by the same token, where I might hypothetically consider an advanced AI to be a full-blown person, a pro-lifer might see it as nothing more than a bundle of cables and microchips). It makes debate pretty pointless, because for the most part we're just hammering away at foundations that for the most part none of us really recognise that we have and which we don't stand much of a chance of denting. All you can really hope for is compromise, and that much is never gong to emerge from a debate between evangelical fundamentalist like CelticEmpire and an epitome of godlessness like Useless.