Recently an anonymous poster has chosen to defend the viability criterion of what is required to be a human being. I take that view to be severely flawed for many reasons, not the least of which are these:
Viability = df the ability of the human fetus to live outside the womb, assisted or not, currently set at around 21-22 weeks.
i) Debate over what it is to be a human try to find those necessary or essential characteristics (or properties) such that if the entity under question did not posses said characteristics they would not be the same entity. In other words, we're looking for essential rather than accidental properties. Stated another way, we might put the questions this way: What would have to be true of entity E at all possible worlds W for it to remain E, and what could be false of E at W such that it would still be E? For example, to appeal to Christian presuppositions for purpose of example, if I was not an image bearer of God at W, I could not be a human being. On the other hand, if I lived in Spain rather than America at W, I would still be a human being. In fact, I'd still be me.
ii) Apropos (i), the viability criterion is an accidental property. It's a measure of the sophistication of neonatal life-support systems. Thus, viability is a constantly changing standard of measurement. A preemie born in a hospital would be viable while that same preemie, at another possible world, born in the deepest depths of a jungle, would not be. Thus this view claims that one's location would determine its humanity. But we saw that this was false in (i). (In fact, if the viability thesis is meant to supply what is essential, then the two preemies wouldn't be the same. But this flies in the face of thousands of years of philosophy by assuming that where you're born is somehow essential to who you are.)
iii) Apropos (ii), viability is not a static concept but a relative one. Something is only viable in respects to something else, viz., it's natural environment. Thus, if you shot me into space, I wouldn't be "viable." I couldn't live in that environment. I would still have a fully developed circulatory system, properly functioning brain, etc., yet I couldn't live in space (without a space suit). I would still be a human being. (If you don't like leaving earth, just substitute being dropped off at the North Pole, or under water.)
iv) Apropos (iii), thus we see that the viability criterion for the humanity of the fetus claims that if the fetus was pulled from its natural human environment for that stage of development and it couldn't survive, it would not be human. But we saw that (iii) renders this notion false.
v) Proponents of the viability criterion need to tell us what it is about viability, exactly, that (magically?) transforms the nature of the fetus from a non-human nature to a human nature.
vi) Post-birth infants are not "viable" as they would soon die without the proper shelter and nutrition needed to sustain its life. Thus the viability criterion cannot stave off infanticide.
vii) How is the viability criterion any less arbitrary than arguments for non-humanity of black people? What standard allows one to simply announce that humanity depends on viability?
viii) Suppose a fetus F was not viable at time t1 but became viable at time t2 such that if F were born premature at t2, F would be able to survive (with assistance) outside the womb. Now suppose that F in fact was not born at t2 but developed a condition at t3 such that F would not be viable if born at t3. The viability criterion therefore gives us the highly counter-intuitive conclusion that F was not human at t1, human at t2, and not human again at t3. This would at least mean that F was not the same entity at t1 and t3 as it was at t2. Therefore, the mother actually carried (at least) F1, F2, and F3.
ix) Actually, many preemies are not "viable" in that they need major assistance in living. They are kept in the NICU for weeks at a time, "kept alive by an array of specialized ventilators, intravenous feeding pumps, and advanced diagnostic gear, as well as round-the-clock attention from the hospital staff." In fact, there seems to be no non-arbitrary, relevant difference between life in the NICU and life in the womb (for some of these). See picture:
Given the logic of our Anonymous poster, these babies are not human and thus if I killed one I should, at best, get slapped with a fine, maybe a year or so in the cooler.