To revise Rene Descartes, "I think that I think, therefore I think that I am."
I may have over-trivialized this rather deep question. It all goes back to Fernand LaPlace's (in retrospect rather idiotic) premise that if we could know the time and place and motion of every electron in the universe, then we could predict with 100% certainty the outcome of the universe at any given point in future-time.
Enter quantum theory, or its ancestor, The Uncertainty Principle. We can either know the location of an atom or its velocity, but never both; the mere act of observation changes the fact observed. This much has been proved to be a fact of reality.
We are coming close to what matters here, IMHO. If it turns out that the particular universe in which you and I dwell should prove nothing more than an involuted hypershpere. and that all my decisions were pre-ordained in an infinite loop of possible universes, then this argument strikes me as a cop-out. What we have at hand is one Actual Universe, in which x+y always equals y+x. That's all we have to go on. In theory, there may be another universe in which this basic truth does not hold true. But frankly, I don't believe it can exist, and here's why: in any such alternate universe, there must exist a redefinition of reality, such that said redefinition includes a saner perspective on the nature of events (i.e. events have causes, which in turn have causes, and projecting forward, events have results, which in turn have results).
I am tempted to apply this logic to foreign policy as practiced by various nations, but for the moment I shall let this contentious topic lie sleeping in its bed.
In the current GOP universe, women are evil, or at best, untrustworthy. Should a woman be "legitimately raped", then and only then should she be entitled to abort. Admittedly, Mr. Romney has retreated from this egregious statement, and even gone so far as to repudiate the speaker's right to wear the Republican flag.
But to be fair in this argument, I also recognize that advances in medical science can cause premature babies to survive, albeit with massively expensive interventions, but in theory that is beside the point. The Republicans are correct, in my opinion, in raising the question, "When does life begin?" Unfortunately, the answers may displease virtually everyone.
Suppose that the Catholic belief turns out correct -- that life begins at the moment of conception. One can imagine that in 20 or 50 years a "baby" created an hour ago could be extracted from its mother and fed via test tubes and such, and 9 months later become an actual baby. A knotty problem, to be sure, but not without the realm of futuristic reason. This could happen, and likely will, within the current century. And then what? What if it can be scientifically proved that one second after the moment of conception, a baby is viable? What does that do to a woman's right to choose?
I don't know. But I can see this battle coming rather than going away. With every new medical advance, the definition of viability grows shorter.
However, there is an alternative, as practiced in ancient Greece and Japan. In the former case, a baby was left alone for three days, and if s/he survived, then s/he became a person. In the latter case, it was held that a child was not a person until s/he achieved six months of age, at which point the soul became seated in the body, and the infant became a Person.
There is an even grimmer side to these arguments: if one is given the right to terminate a child deemed not yet a person, then why should this right to terminate not also extend to people no longer deemed to be persons? I'm thinking of the victims of Alzheimer's and similar diseases. I'm deeply uneasy with that, but as yet I'm unaware of a logical argument against this extension.
What is a Person? Is an acorn an actual oak tree or merely a potential oak tree? Is a 300-year-old oak tree to be considered nothing more than the fuel for the next forest fire? I don't know. I have seen some thousand-year-old trees and deemed it the worst sort of violence to cut them down. I have seen and swam with turtles two centuries old, and the thought of their becoming soup repels me. Both these citations work from a simple principle: what right do I have to kill anything older than me or for that matter, my parents? Any plant/animal that can survive more than a century ought to be valued solely on those terms. You and I have relatively brief life-spans. Numerous other species dwarf our expectations of longevity, regardless of medical advances.
And so let us leap to someone's imagined future, in which Immortality is possible (although affordable only by the ultra-rich). As one S-F writer expressed this, "What no one told us about immortality is, How many assholes you'd meet in 600 years."
I don't know about either of these questions, but I do think that we need to consider both.