It is obvious that both the female egg and the male sperm have a form of life but no one would call either a person. When they unite to form a zygote it certainly is alive. There is a potential person there and a great many people would claim that it actually is a person. Let us grant for a moment that the zygote is a person and let us call that person Mary. I choose a female name since all embryos are female until about the sixth or seventh week.
Now, we all know that a zygote develops into an embryo through the process of cell division. Every now and again the first cell division does not produce a two celled embryo but rather a second zygote --- identical twins. Did Mary suddenly become two persons? Was Mary two persons to begin with? Was Mary even a person to begin with? Let us set those questions aside for the moment and grant that the second zygote is also a person whom we shall call Margaret. It is entirely possible that one or both of these zygotes could divide again to result in triplets, quadruplets, quintuplets etc. The same question applies as to whether one person can became two, three or more persons. When does a person become a person?
These questions might be difficult enough but now it becomes even more complex. Sometimes two eggs are fertilized to form non-identical twins. Once again, let us call them Mary and Margaret. Rarely the two zygotes merge together again to form a two celled embryo. This is called a chimera. Who is this new embryo? Is it Mary or is it Margaret? This new embryo, this chimera, let us call it Mary, develops to term and is born. There is now no question at all that Mary is indeed a person. But here is the odd thing, some of the organs of Mary carry her genes but other organs carry the genes of her twin sister Margaret. So Margaret continues to exist within Mary or perhaps it is Mary within Margaret. Do we have two persons within a single body?
These very serious questions of person-hood arise only if we assume that the soul is infused at conception and that the brand new zygote is fully a person. Is there a more reasonable understanding? I believe there is. Personally I believe that the developing fetus becomes a person only when it is able to survive outside the womb. Sentience occurs at about the same point in the pregnancy very late in the second trimester. For this reason I am against abortion beyond the twentieth week except in very rare extreme circumstances.. Otherwise I believe that abortion should be legal, it should be safe, it should be available and it should be the woman’s informed choice but most important of all --- it should be rare. In conclusion, we should always keep in mind that there is no more powerful abortifacient in the world than poverty.
