And not a human being.
I remember when I was kid, my mother was making eggs when suddenly she found a chicken fetus in one of them.
I remember feeling this sense of sadness and regret, feeling sorry for a life that had been cut short before inception. Even my childish mind knew that was a chick in the making, not an inert clump of cells. And we certainly didn't eat that egg, even thought the egg was still mostly yolk.
Because atheists do not believe in vertical causes, because they do not believe in such things as forms and archetypes, they have no understanding that a human life is the whole extend of the existence of a person, both in space and in time. Every fetus has already plotted ahead of itself the entire gamut of the existence of a human life, so that killing the fetus is to kill the whole person.
If the essence of a being could be determined by the state of a given creature in time as opposed to the form of that being, the form including within itself the entire gamut of the possibilities of that being regardless of it's current state of development, wouldn't killing a baby or a child be less morally reprehensible than killing an adult person? After all, a child does not have the same level of cognition than an adult. One would be compelled to ask whether killing a baby should be seen as any more reprehensible than killing a pig or some other kind of animal we consume for food. After all, by any measure the pig demonstrates an higher level of cognition than a human baby. Why then do we bulk at the idea of killing a baby but killing a pig presents no problem for us? Could it be because we know that the baby is a human being regardless of it's state of development?
What makes modern abortion even more reprehensible is that the whole thing is treated as a simple matter of convenience. Modern abortion today is done purely and exclusively in name of utilitarian materialism. Abortion for serious causes, say, to save the life of the mother for instance, may be permissible in the same sense that killing in self defence is permissible, but neither exceptions undermine the seriousness of the act. One must kill or be killed in war but just because conditions like that exist in the world does not mean we have the right to turn murder into a casual matter of convenience, but this is what is being argued when it comes to abortion, and sometimes even murder as in the case of euthanasia of the old or the infirm.