I define viability as the ability to live outside the womb without breathing tubes and extensive medical equipment to act as artificial womb. I'm not sure exactly how many weeks that is. The ability to keep it's own body alive proves it graduated from fetus to human being.
So, let's see if I understand your logic. According to your definition, all those "fetuses" that are born pre-mature and need help with breathing, etc. don't qualify as "human beings" until they're developed enough to be weaned off the technology that helped support them, right? So those (oh, I'll use my term this time) BABIES aren't human beings until the doctor signs off on letting them be released from the hospital, huh? In other words, by your definition, a baby that has been born prematurely is still a fetus and not yet a human being, correct? Birth no longer is a defining point, now it's not being technologically-dependent on life-support? The fetus has to "graduate" from it's dependence on the womb in order to qualify as a human being? I am stunned!
So, it's okay to kill it when it is at its most vulnerable. Nice.
That's a decent definition of viability, but it doesn't have anything to do with personhood or the fact of being a human being. To "choose" viability as the beginning of personhood is a *personal choice,* one which has no validity in science and may be countered by the next person's personal choice. It is no standard.
On the other hand, we have science, which defines the beginning of life at fertilization. We know the species, we know whether or not the cell is alive, and if anyone has the right to protection by the rest of us to ensure that he or she is not to be killed, all members of the species have that right.
Otherwise, we're left with the whims and personal opinions of individuals who can sway the group according to their own prejudices, wants and immediate needs.
There's no justice in using personal or discriminatory definitions or opinions to define who will live and who will die.
Put a two-year-old out in the wild with no human intervention. According to the above statement, a two-year-old is not a human being because it can't keep it's own body alive for any significant length of time without human intervention, right? Conclusion: killing a two-year old is okay, correct? And don't give me the "artificial means" excuse. Either the child is capable of living without "human intervention" or it isn't. Don't impose a bunch of artificial conditions in order to find a loophole for infanticide.