It is the key element to the whole of the debate. The premise being that if life begins at conception, then all abortion is murder. This is the pro-life position If life does not begin until delivery, then abortion is simply the removal of an inconvenient bit of parasitic tissue. This is the pro-choice stance.

The abortion debate is a nearly perfect demonstration of the limitations of concept: both sides have taken a complex phenomenon and tried to squeeze it into a tiny set of boxes that are too small to hold even the head. "What is the definition of life?" is one of those boxes. "What is a right to life?" is yet another, and "what are the limits of personal rights over one's body" is a third. Concepts are generalizations and simplifications: we take an infinitely complex Reality, and carve it into pieces with our thoughts. Then we label those thoughts, and start moving them around like pieces on a game board... "this is like that, so therefore these rules should apply to that", etc. This is what happens ad nauseum in the abortion debate. A fertilized egg is a little bit like a human being. A nearly-born fetus is a LOT like a human being. Human rights and the "right to life" are inventions of thought and language, not something which exist independently of our ability to create concepts. Individuals should not have unlimited rights to do whatever they please to their own bodies. Those are all statements which challenge the fundamental simplifications at work in the abortion debate. Until those simplifications are stripped from the argument, it will rage on in endless turmoil, with no resolution possible.

The current pro-choice argument is that abortion can't be murder before life begins, and therefore abortion should be allowed. Pro-choicers tend to believe that life begins at viability or birth. Pro-lifers disagree and say that life begins at conception, therefore abortion should never be allowed. I think we need to take a different perspective, one in which the definition of life isn't so important. Nobody has an absolute right to survive under all circumstances; otherwise, we could never have capital punishment. We need to accept that there are circumstances when the fetus's right to survive is overruled by the mother's right to bodily integrity. Obviously we would need as a society to define those circumstances.