Identical Twins & Souls

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.

Author: Jimmy Akin

Jimmy was born in Texas, grew up nominally Protestant, but at age 20 experienced a profound conversion to Christ. Planning on becoming a Protestant seminary professor, he started an intensive study of the Bible. But the more he immersed himself in Scripture the more he found to support the Catholic faith, and in 1992 he entered the Catholic Church. His conversion story, "A Triumph and a Tragedy," is published in Surprised by Truth. Besides being an author, Jimmy is the Senior Apologist at Catholic Answers, a contributing editor to Catholic Answers Magazine, and a weekly guest on "Catholic Answers Live."

29 thoughts on “Identical Twins & Souls”

  1. It’s a no brainer. God knows the actual future with infallible certainty & thus logically can create two souls for a single zygot destined to become twins.

  2. Thanks Jimmy,
    It seems to me that #1 is more compatible with a loving God who wouldn’t want a soul to die needlessly. It also seems to me that in a unfallen world, where twinning would never lead to the death of one or both of the twins, that this type of twinning would be another echo of the Trinity (one top of the echo already there through procreation between husband and wife). It seems that chimerism is part of a fallen world, and that God again would try to preserve at least one of the souls in a case like that. But those are just my thoughts, I may be looking to deeply into God’s intentions – what I think he would do.
    BenYachov: That’s what I thought too, but it seems that classical theology and philosophy precludes two souls sharing the same body – unless this doctrine has not been infallibly defined it seems that the second soul has to be created at a moment post-conception. Maybe the defintion of “conception” itself needs to be reworked in such a case.

  3. “It seems to me that #1 is more compatible with a loving God who wouldn’t want a soul to die needlessly”
    The flaw here is your suggestion that the soul dies. Once created, the soul never dies, it will ultimately reside in heaven, hell, or possibly (improbably in my opinion) in some form of limbo.
    God allows many millions of people to die, it is never “needlessly”. While He does not necssarily desire it, but He permits it for the greater good.

  4. The first explanation that came to my mind is Ben Yachov’s theory for two souls inhabiting the same zygote until splitting. Is there a reason why this would not be the case?
    It seems that phenomenon of demon possession demonstrates the possibility that one body can be inhabited by two spiritual entities. So I am not sure why this is not the way it could happen with twins as well.
    Interesting post!

  5. “I thought I’d answered this on the blog before”
    That must’ve been your evil twin.

  6. Hi Matt,
    I’m sorry, I should have been clearer – when I used the word “die” I was thinking of the soul being seperated from the body, rather than annihilation (which you rightly point out does not happen). It seems to me that this is how the soul experiences death.
    Also, it seems that since Being as such is good, death an evil, and that our souls are enriched by lived experience, it would be better for the first soul to be preserved. It would also seem to be a following of Ockham’s Razor as well.

  7. Dr. Jerome Lejeune cited evidence that the embryo prior to twinning was already programmed to split. In his view, therefore, the body was a two-embryo body even before the split when it was only one. If that is true, could not that body contain two souls?

  8. StubbleSpark: a demon posessing a body does not (and cannot) have the same relationship to the body as its human soul, however. Human souls are a different sort of spirit to the angels, as the former are inherently forms of material bodies whereas the latter are not.
    I’m also not sure that Anselm’s approach (essentially suggesting that in that case there are two bodies) is satisfactory, since embryos can be twinned by external factors (e.g. manual intervention in embryonic cloning).
    So, we’re back to the question question of whether two human souls can be forms of the same body.

  9. I wonder if there isn’t a third possibility other than (1) and (2). And this is simply to consider the possibility that a single human soul can split into two distinct human souls without itself continuing its existence in just one of them or being destroyed/disembodied entirely.
    Suppose Embryo B grows up and is shown a scan/ultrasound image of Embryo A. Would it be incorrect for B to answer “Yes” to the question “Is that you when you were an embryo”?
    This goes back to the classical conception of being as essence and existence. History falls under existence – and it’s clear that both B and C have a common history (and, therefore, existence) in A. Why not consider a common essence (soul) as well?

  10. BenYachov: That’s what I thought too, but it seems that classical theology and philosophy precludes two souls sharing the same body – unless this doctrine has not been infallibly defined it seems that the second soul has to be created at a moment post-conception. Maybe the defintion of “conception” itself needs to be reworked in such a case.
    Ryan,
    Maybe we have to rework our understanding of the matter of cells. It may be possible that what we think of as one cellular entity may be two at what is now an undetectable size.
    ????

  11. MenTaLguY–I don’t think that an external intervention that causes twinning undercuts my question. An individual embryo who later is acted upon externally to cause a twin was likely one body and soul to begin with, and it was the external cause that produced the second body and soul. In contrast, an embryo that is programmed to twin (if this happens) could be seen as having a different kind of body than an individual embryo, even an individual embryo that is later twinned externally. So I still wonder if an embryo programmed to twin could be, in a sense, two bodies and therefore could possess two souls.

  12. I think that #1 is most likely. This would require flexing the definition of “conception” to mean the point in time at which the embryo comes into existence. Thus Embryo 1 receives a soul at the moment the egg and the sperm are united; at the moment Embryo 2 comes into existence, God gives it its human soul.
    As to demon possession, the soul is part of the human person; a demon is not. A human cannot exist without a soul – you just have dead flesh. A demon, as an angelic spirit, has the ability to act upon persons, but does not become identical or inherent to the possessed (whether they act upon a person’s body directly in a manner analogous to the human soul, or whether indirectly by influencing the faculties of the mind or soul, I must confess ignorance).

  13. “I’m sorry, I should have been clearer – when I used the word “die” I was thinking of the soul being seperated from the body, rather than annihilation (which you rightly point out does not happen). It seems to me that this is how the soul experiences death.”
    I don’t know where you might come to this conclusion…. the soul is the person’s essence, so soul experiences what the person experience. Being as such is good, except perhaps for Judas. That has nothing to do with death.
    Death is a result of the fall, but it is not evil in itself, nor does God necessarily cause the death.
    “Also, it seems that since Being as such is good, death an evil, and that our souls are enriched by lived experience, it would be better for the first soul to be preserved. It would also seem to be a following of Ockham’s Razor as well.”
    Once created, a human being will exist for all eternity. For a time, the body will return to dust, later to be restored, but the soul always will exist.

  14. “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,”
    If by “classical metaphysics” Jimmy means “Thomistic metaphysics,” this is accurrate. However, the question of the “plurality of [substantial] forms” was a matter of vigorous debate in the middle ages, with a number of Big Names coming down on the other side. However, “substantial form” is not synonymous with “soul”, and the claim probably still holds for souls no matter what you think about substantial forms in general.

  15. perhaps the problem lies with the pagan Greek metaphysics. Soul is the translation of nephesh – breath, not of ‘adam’ for dirt.

  16. If the soul is the formal cause of the person, we would have to decide whether a zygote is one person or more throughout its totipotential phase.
    I would argue the gestalt theory that each combination of totipotential cells is its own person, and that daughter zygotes obtained by induced fission would be substantially and philosophically distinct entities from the mother zygote from which they were created. In that case, we would conclude that the soul of the mother zygote was also distinct from either of the souls associated with the daughter zygotes. Precisely what relation those daughter zygotes’ souls would have to the mother zygote’s soul is an imponderable mystery, but I do think we have to consider strongly the case that the mother zygote is conceptually different from either daughter.
    This leaves the difficulty of deciding when conjoined twins manifest separate souls, since often the fission in incomplete (and, in fact uncompleteable), but I am fairly well convinced that the importance of the zygote’s humanity argues for the unique dignity of both mother and daughter zygotes.
    PVO

  17. Ryan C,
    Since the Church has taught that human dignity demands a child be concieved through natural married love (for instance in Donum Vitae I highly doubt any identical twining is in the plan of God, therefore I do not see why case 1 is any more likely than case 2. The death of embryo A could be a tragic medical accident just like miscarrages or chimerism.
    Also, I do not see twining as a reflection of the trinity. Two persons coming together in love and thus bringing forth a third person is a reflection of the trinity. If that third person splits giving rise to a fourth person I do not see how that reflects the trinity, it involves either four persons or two depending on your “starting point.” If the first embryo dies then you have two persons coming from one which works except it is a horrible abomination involving the death of an innocent person (not that there is anything wrong with the twins produced themselves) which could not have been ordained by God.
    Puzzled,
    It’s all right for non-Catholics to post here of course but please don’t go trollish on us and start just attacking the Catholic faith (for example by attacking our essential use of Greek metaphysics in defineing doctrine). I know you are just edging into this area with your post but if Greek metaphysics is invalid then so are many if not most of our doctrines. Also, just because the Jews understood the soul in a certain way (as “breath,” like so many other ancient peoples) does not mean that the Greeks could not or did not come to valid conclusions about the soul themselves, that in some ways went beyond what the Jews knew.
    About whether the original zygote could have two souls, I think this is absolutely impossible. The soul is the form of the body by which it lives. I believe Bl. John Duns Scotus proposed there could be two forms of the body, one that remains after death (the basic, constantly changing form of the physical body, if I follow him) and one that is spiritual and specifically associated with life which leaves the body at death. In any case a body can have only one such spiritual form. What would be the “extra” soul be the form of, or would one body have two of the same kinds of form? It doesn’t work out anymore than saying that the original zygote is two identical bodies in one.
    What I really wonder about is conjoined twins. Especially ones that share so much of the same body that they almost simply have two heads. Yet as long as they have their own, independantly operating brains they certainly seem to be two distinct people. Could it be possible for them to have the same soul and the two “minds” is an illusion from having two brains? Or does the soul account for enough of the mind (I have always been confused by the subject) that if there seem to be two minds then there are definitely two souls? If that is the case does one twin’s soul (or rather the portion of the body that her soul pertains to) end at some point and the rest of the body is the other twin’s. What if both twins equally feel and control the part of the body (say, the legs) that they have in common, as I think is sometimes the case?
    Especially problematic, in light of the connection between sustained life and the soul, may be the case of one twin entirely dependant on the organs of the other for survival.
    Further, what about “parasitic” twins, where the other “twin” is just a small part of a body. When it is an extra limb or something like that it seems clear to all that it is not a distinct individual. Yet what about when it is a face and brain, as I have seen on TV. That “parasite” which was removed from the back of an otherwise normal toddler girl’s head was clearly not all there mentally but at least tended to smile when someone looked at it. I would be more inclined to call that an actual human being, yet what is the real difference between that and an extra limb, since the soul resides in the whole body equally, not just the brain or head. Also it seems like there is only a difference of degree between “parasitic” twins and more complete conjoined twins, so what does that mean for either? I am quite confused.

  18. Anonymous,
    Why not? If material objects can be divided, then why can’t their substantial forms (which is what the soul is for a human being) also be divided?

  19. Stoodley:
    “two persons coming from one which works except it is a horrible abomination involving the death of an innocent person (not that there is anything wrong with the twins produced themselves) which could not have been ordained by God.”
    The death of a person is not an abomination unless it is murder. When a human dies by natural causes, be it young or old it is natural and just, yet it may be sad for us left behind, and difficult for us to understand. It seems that most such deaths are in accord with God’s permissive will and so not directly ordained by Him, but permitted.
    Kensy:
    “Why not? If material objects can be divided, then why can’t their substantial forms (which is what the soul is for a human being) also be divided”
    In order for a thing to be divided it must have parts, some parts go here, some go there. The soul of a thing has no parts, therefore it cannot be divided. Just because we can’t understand how the soul is in regard to twins, and chimera, doesn’t meant that the fundamentals (such as the indivisibility) are wrong.

  20. Matt,
    Thanks for responding.
    First – where does the Church teach that human souls are indivisible? I am aware of the *philosophical* conception (I don’t know if it is official doctrine) of God as simple, but I have never heard it said about the human soul.
    Second – the human body clearly has parts (e.g. hands, feet, head etc.). As beings, each of these parts would also have substantial forms. It would seem to me quite natural that these substantial forms would be related to the substantial form of the human being as a whole (i.e. the soul) in the same way as they are related to the body. If, as you say, the human soul does not have “parts”, then how are these forms related to the soul?
    Thanks in advance.

  21. Hi J. R.,
    I stand by my original assertion. While it is clear that frutful marital love is the primary sign of the Trinity in our world, it is not the only one. The image of God stamped on us is not an algebraic equation that has to balance out exactly, but more an “artistic” pattern that repeats itself in various ways. The relationship between those in the Body of Christ is Trinitarian, for example, and the call to the marriage feast is the call to enter into the inner life of Love of the Trinity.
    Anytime I love the Other as I love myself (as Christ commanded) I manifest this Trinitarian ideal. Indeed, if I am giving selfless love to another, it is only because of the Holy Spirit indwelling in me and giving me the grace to do so. If I do not love selflessly, as John says in his first epistle, I am not in the light, which is the light of the Trinity. It is not just that we love God because he first loved us, but also because God first loved us that we are able to love each other. All love finds its source, then, in the Love borne between the Father, Son, and Holy Spirit.
    Twins are known for often having a very deep bond with one another. It is also true that they share an intimate physical or substantive connection, even if they are not identical twins and simply share the same womb (a fact of life that also has a deeper spriritual signifigance). Based on these reflections, I think it’s fair to see the deep love that exists between twins who are united so intimately in body and in soul (since God creates their souls so closely together in time) as a sign of the Trinity, and twinning itself as a part of the divine plan – part of God’s plenitude.

  22. Kensy:
    “First – where does the Church teach that human souls are indivisible? I am aware of the *philosophical* conception (I don’t know if it is official doctrine) of God as simple, but I have never heard it said about the human soul.”
    A pure spirit takes up no space
    A thing that takes up no space does not have parts.
    A thing that has no parts cannot be divided
    Therefore, the soul can’t be divided
    Are any of these points is not self evident?
    Catholicism is based on Faith and reason. The philosophical points are widely held by Catholic philosophers from the beginning of the Church and have never been contended. I don’t know if there’s any Magisterial document which can be cited, it’s probably in a Catechism (CCC), or perhaps the Baltimore.
    “Second – the human body clearly has parts (e.g. hands, feet, head etc.). As beings, each of these parts would also have substantial forms. It would seem to me quite natural that these substantial forms would be related to the substantial form of the human being as a whole (i.e. the soul) in the same way as they are related to the body. If, as you say, the human soul does not have “parts”, then how are these forms related to the soul?”
    The parts of the body are not beings, they are just that, parts of a being. If they are separated from that being, they are no longer associated with the soul of that being.
    Keep in mind that the human soul is distinct in that it is spirit as well as soul (essence).

  23. Matt McDonald,
    Human death is not in the “original” plan of God. I mean his ordained plan apart from all sin and evil, in one sense everything that happens is really part of God’s plan, with Him allowing evil so that greater good may come about.
    Still, death is the result of Original Sin. Perhaps I was wrong to call it an abomination, since that word suggests moral evil, which I suppose ordinary death does not have since it is not a sin. It is a result of sin. Still, the fact that it is a result of sin means twining by possibility 2 and chimerism could not be in the original plan of God.
    Ryan,
    I still don’t see twining by possibility 1 as being in the original plan of God. It would involve a human being coming into being in a way that at least is a far lower and less clear reflection of the trinity.
    Yes, there are many reflections of the trinity in Creation, from marriage to the Church to rain or sunlight falling to the earth, giving life. All forms of love are also closely connected to the Love of the Trinity.
    Twining, however, does not involve any love at the time it happens and has no clear relation to the number three. It is true that one person comes from another, so perhaps it could be interpreted as a reflection of the relationship between the Father and the Son, but this is very obscure and inferior to the form of conception that the Church teaches is the right of every child. It is essentially a form of cloning
    It is true that Eve came about in aproximately the same way (formed directly from the body of Adam, however God did that). However, this reflection of the relationship between the Father and the Son was destined to be completed with the formation of a third person from the love of the two. In the case of twins there is certainly no divine intention for this, or as yet (thank God) a practical possibility for it.
    I will grant though that a special relationship does tend to form between twins, and though I am not sure this is stronger between identical twins than between fraternal twins. In any case I suppose this is a good that God brings out of the process. Still, I hold it could not have been in God’s “original” ordained plan (which he never intended to work out anyway). It required original sin to bring it about, like so many other goods.
    Besides, though I do not know the details doesn’t identical twining come about through a mess-up in the developement of the original embryo? Such biological mistakes could not have happened without original sin, even if they result in good things (like how in my opinion the mutations and natural selection resulting from original sin brought about ethinic diversity, which is a great good).

  24. Stoodley:
    “I still don’t see twining by possibility 1 as being in the original plan of God. It would involve a human being coming into being in a way that at least is a far lower and less clear reflection of the trinity.”
    Ok, you got me, I didn’t realize you were arguing from the perspective of a non-fallen world. I was arguing from the perspect of a fallen world. Mind you, arguing from the perspective of a non-fallen world seems a little pointless don’t you?
    For everyone’s benefit, unless I say otherwise, I am always arguing from the perspective of a fallen world, and that when I say death is not sin, or a result of a sin, I’m referring to immediate sin, and not the original sin which caused all death.

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