A reader writes:
I’ve been discussing the issue of ensoulment and personhood with some people
and a person brought up an interesting question: When it comes to identical
twins, which come from the same fertilized egg, when do the two souls and
thus the two persons come into existence?I was under the impression that
the Church held that God creates each soul at the moment of conception, but
the splitting off of twins suggests that in this case, he might create the
souls later at the point of division.Or is it possible that he creates two
souls to share the zygote until they split?Anyway, I figured you might be
a good person to ask this question.
I thought I’d answered this on the blog before, but Googling my archives didn’t turn it up, so here goes . . .
The soul is the substantial form of the living human body, and so any time you have a new living human body, you have a new soul (the resurrection doesn’t count, since that’s not a new body; it’s a resurrected old one).
It is very difficult to see how a single living human body could have two substantial forms. Indeed, classical metaphysics would say that this is impossible by definition, so I’m not inclined to go that route in explaining what happens in twinning.
Normally a new living human body comes into existence at conception, so that’s normally when the soul comes into existence as well, but the phenomenon of identical twinning indicates that the situation is more complex than that.
It would seem that there are two possibilities. Either
1) Twinning occurs in such a fashion that Embryo A fissions off a new embryo, Embryo B, without losing its identity as Embryo A. (This is analogous to the way in which a Adult A could have a clone of himself made from a skin cell without losing his identity as Adult A.)
2) Embryo A fissions in such a way that neither resulting embryo can be said to be the same entity as Embryo A, so there are two new entities, Embryo B and Embryo C. (Imagine taking Adult A, splitting him down the middle, and regrowing the missing part of the body on each resulting half so that neither resulting individual has a greater claim than the other to being Adult A.)
In case (1), it would seem that Embryo A received his soul at the time of conception and Embryo B received his soul at the time he fissioned off from Embryo A since that was when Embryo B’s body came into existence.
In case (2), it would seem that Embryo A received his soul at conception and that Embryo A was a short-lived individual who died when he fissioned into Embryo B and Embryo C, both of whom received their souls at the point of fissioning.
Now, just for the sake of completeness, let’s talk about the opposite of twins: chimeras.
As before, there are two scenarios:
3) Chimerism occurs in such a fashion that Embryo A is so large that it absorbs Embryo B without becoming a fundamentally different entity. (This is analogous to Adult A having a heart or kidney transplant from another adult; the minor addition of cells from the other adult does not turn Adult A into a fundamentally different person.)
4) Chimerism occurs when Embryo A and Embryo B merge in such a way that the resulting entity is neither one of them but is a fundamentally new entity, Embryo C. (This is kinda like the Tuvix episode of Star Trek: Voyager, only on the cellular rather than the DNA level.)
In the case of (3), Embryo A received his soul at conception and continued in existence. Embryo B also received a soul at conception but then died when he was absorbed by Embryo A.
In the case of (4), Embryo A and Embryo B both received their souls at conception and both died when they fused into Embryo C, who received a soul at the point of fusion.
If you want to be extra complete, you can posit the case of identical twins who then fuse to become a (genetically undetectable) chimera, and you can run the combinations on that one yourself, them being a combination of scenario (1) or (2) with scenario (3) or (4).
You could also think about what would happen if a chimera then twinned. Or if twins chimerized and then twinned again, etc. It’ll all just be combinations of (1)-(4), though.
Because of difficulties in determining when a new individual has come into existence, it could be hard or impossible to distinguish between scenario (1) and (2) or between scenario (3) and (4) in practice, but these would seem to be what is happening, even if we cannot make the determination in a particular case due to the limits of present doctrinal development on the subject of individual identity.
