The first is called the “immediate animation, immediate ensoulment theory”, which states at the very moment of animation (when life begins), the newly conceived human is animated by a rational soul.
The second theory is called the “immediate animation, delayed ensoulment theory” which states there is a progression from vegetative to animal to human soul as the principle of animation. This theory was based on Aristotelian biology and was popular in the middle ages. St. Thomas Aquinas also agreed with this theory and stated he believed males received their soul 40 days after conception and females after 80 days. It has been revisited and refined recently. This is a summary of the new “modern” theory of delayed ensoulment:
First because the soul is the substantial form of the body, the rational soul cannot be present until there is a body present that is significantly complex and organized to receive the soul. Second, a formal cause is present only in a finished product. An actual human soul cannot be united with a virtual human body. Third, there is no human body in the zygote. Fourth inasmuch as all the positive features of the human body derive from the soul, until the soul is present there is no human being. (taken from the article above)The article that explains the two theories does a wonderful job explaining why the second theory isn’t reasonable. I would also argue that the first makes sense from what we know about God and us: That we are in existence because He loves us, and he loves us because we have human souls (which He of course created). I would think then that as soon as we exist (in the smallest sense, or right at conception) then God has loved us into existence therefore confirming we have a soul. Why would God create us and then wait a little while to create our soul? Can we have a body without a soul? Again I would say not while we are living, and a newly conceived human is definitely living.
Why does this even matter?
If a fetus (I mean fetus as any stage of development before birth) doesn’t have a soul, then it could be argued that abortion is acceptable: since there is no soul yet, they aren’t really human, and thus also have no rights. The response to this may be that even with uncertainty, it’s always better to err of the side of life (meaning don‘t ever abort because you can‘t know for sure). However, there may even be a better reason why abortion is still gravely wrong even if there is no soul present:
Some scientists and philosophers will attempt to argue that if an early embryo might not yet have received its immortal soul from God, it must be OK to destroy that embryo for research since he or she would not yet be a person. But it would actually be the reverse; that is to say, it would be more immoral to destroy an embryo that had not yet received an immortal soul than to destroy an ensouled embryo. Why? Because the immortal soul is the principle by which that person could come to an eternal destiny with God in heaven, so the one who destroyed the embryo, in this scenario, would preclude that young human from ever receiving an immortal soul (or becoming a person) and making his or her way to God. This would be the gravest of evils, as the stem cell researcher would forcibly derail the entire eternal design of God over that unique and unrepeatable person, via an action that would be, in some sense, worse than murder. The human person, then, even in his or her most incipient form as an embryonic human being, must always be safeguarded in an absolute and unconditional way, and speculation about the timing of personhood cannot alter this fundamental truth. -Fr. Tadeusz PacholczykNow although there is no way to be completely sure when exactly ensoulment occurs, I think we can follow the evidence and, through reason, come to a pretty sound conclusion. Personally I have trouble with the idea that ensoulment doesn’t occur at conception. For one, we know that a new human was formed, with it’s own DNA and the like, and I would argue that when God created that new human, why would He not also create it’s soul as well?
What about twins?
In the case of identical twins, the zygote is split after conception. Some zygotes split in 1-3 days (with 2 amniotic sacs, 2 placentas). Some split in 4-8 days (2 sacs, 1 placenta). Some split in 9-13 days (1 sac, 1 placenta) and some split after 14 days (usually conjoined twins). How can one say that ensoulment occurs at conception when later, the zygote can split and become two babies? I don’t see this being a real problem, and I have two theories that reconciles this to ensoulment at conception (I like the first one best):
1. Is it not possible that two souls can occupy the same matter for a time, when God knows they will spilt into two separate bodies? And if not, then what about conjoined twins? They are technically one body and yet obviously two distinct persons. I don’t think it’s unreasonable to say that two souls were created in one “body” at first, when God (who also created their bodies) also knew that it would spilt, and therefore need two souls.
2. If the idea that two souls occupying the same body is wrong, then I would say that when the zygote splits, the second baby is then “ensouled”, at the moment of separation. Now it is impossible to discern which baby had the “first soul” and which got his later, but this theory would solve the problem of two souls in one body (if a problem exists).
*Sidenote: I also don’t see a problem with two souls being in one body when I think of examples (other than conjoined twins). A possession is when a demonic spirit enters a body. Would that not be two souls (although not both human) in one body? This is another reason I favor theory number one.
This verse in the bible may also shed some light. It’s Jeremiah 1:5:
Before I formed you in the womb I knew you, before you were born I dedicated you, a prophet to the nations I appointed you.
So on this All Souls Day, may we remember all the souls, whenever they may animate our bodies and whenever they may leave this earth for true happiness.