A priori potential has too much extension to be considered. Do flies have a potential to evolve into sentient beings? Theoretically, yes. So we'd better all start dressing like Jains.innerbrat wrote: But brain development continues after birth - a neonate does not possess the mental cpacity or complexity of a grown adult.
The non-religious arguments I've heard (from my Mum) revolve around the potential for human life.
Ex post facto potential is another matter - And that's why it's wrong to treat the mentally ill, sick, etc, like animals, as the Nazis did. Since their sentience had already been demonstrated the fact that they don't have it at the moment is irrelevant.
So I don't think your Mum's argument presents a serious question to the right of abortion. Until something has been demonstrated to the senses it is not a fact - it cannot be a fact - and theoretical potential does not have a bearing on morality. Demonstrated potential does, however, since the person has already proven sentience and it exists as a matter of record.
Furthermore - One cannot apply the demonstrated potential of the species to a single individual in this case. You are dealing with the question of the morality of terminating a single creature, and so the question comes down to if it has a thinking and reasoning, a self-aware mind. Each such mind - being self-aware, must be treated as an independent object in terms of analysis for theoretical or demonstrated potential.
In that context, the line of seperation is that of self-awareness. When does it begin? Before that, the fetus is nothing more than an animal (in philosophical terms - I apologize if I'm being quite blunt, but I'm cutting through a lot of junk that's built up around this issue) and can be dealt like one. After that the fetus is human and must be accorded the rights of one.
It's entirely possible that the Roman practice of infanticide might in fact be morally legitimate from an absolute standpoint, you're correct, if rather heartless. However, I think research would suggest that - with the beginning of brain patterns indicative of dreaming around the start of the third trimester - we would be advised to consider the fetus as capable of, if very simple, still broadly human thought at that point in time, and thus deserving of the rights and protections of a human being.