What is Life?

I was teaching a class over the weekend and we spent a few minutes discussing some controversial subjects. Abortion was tangentially related to our main subject and one of the students suggested that abortion was ok, because "it doesn't have eyes." Because it was off-topic, I let it go unanswered, but comments such as that make me wonder about a person's definition of humanity. Is a blind person less than human because his eyes don't work? If a person loses one eye, is he only half-human? I feel certain that an extended conversation with such a person would reveal that they have a much more complex definition of humanity than the existence of eyes and/or sight. But to even try to boil it down to something that simplistic seems to me to be absurd. (Cats have better eyes than humans, especially in the dark, does that mean they are better than humans and should never be killed? Or never killed at night?)

This question of "what is life" is being explored and debated by science at both ends of life. It used to be that no heart beat was death, but we now regularly stop and re-start heartbeats. Medicine will try to talk about "brain death," but even that gets complicated as brainstem functions may continue when higher-level functions appear to have ceased.

And I see some logic in trying to make a parallel between the two ends. If a heartbeat signals the end of life, why not use it to determine the beginning of life? If a particular level of brain activity signals the end of life, why not use it to determine the beginning of life? On the other hand, I can't imagine deliberately killing any human life after one has heard a heartbeat or seen a sonogram picture. And I certainly believe it is wrong even before then, though I can understand that the level of abstractness prior to that may make it easier to justify to ones self.

As someone who is intimately responsible for another life right now, it seems to me a serious responsibility. Why would I want to end someone else's opportunities in life for my own convenience??