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.
Jack's post here is the most luminous post on this thread, but its value is enhanced if it is connected with the actual concept of personhood presupposed by Scripture. My thesis is this: A preexistent soul (person) finds its way into a newly formed embryo when that embryo becomes capable of experiencing sensation. Consider these 5 points:
The Bible implies the doctrine of the preexistence of the soul:
(1) "Why was this man born blind, because of his own sins or (John 9:1-2).."
The disciples' question implies a belief that the man might have been born blind because of sins his soul committed prior to birth. Belief in reincarnation was not an option in first century Palestine.
So the disciples themselves presume the doctrine of the preexistence of the soul and even the possibility that soul might have sinned in its preexistent state. That possibility implies the personhood of the preexistent soul. Notice that Jesus never refutes these assumptions, only their relevance to the blind man before then.
(2) This view implies the possibility of the soul's developed goodness prior to b birth. Belief in this possibility finds prior expression in Wisdom of Solomon 8:19-20 in the Catholic OT: "A good soul fell to my lot,
or rather, being good, I entered an undefiled body (Wisdom of Solomon 8:19-20)."
(3) If character development can develop prior to birth, Jeremoah 1:5 takes this one step further by implying that Jeremiah's prophetic call is worked out during his soul's preexistent state: "Before I formed you in the womb,
I knew you and before you were born, I consecrated you; I appointed you a prophet to the nations."
(4) In this respect, Jesus' disciples seem to embrace the Jewish consensus in late antiquity. The process of ensoulment is understand this by the Essenes:
"The soul is imperishable and immortal. Emanating from the finest ether, these souls become entangled, as it were, in the prison house of the body, to which they are dragged down by a sort of natural spell (Josephus, JW 2.8.11)." Philo and rabbinic Judaism agree with this view.
(5) And when are preexistent souls created? The first century Jewish view is expressed in 2 Enoch 23: "All souls are prepared for eternity, before the composition of the earth."