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Conception of identical twins and ensoulment

Started by Kephapaulos, July 04, 2017, 10:33:21 PM

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This was an interesting thread on Suscipe Domine forum:;boardseen#new

How do we explain identical twinning in relation to ensoulment?


God creates a human soul when the matter (the body) is property disposed to be united with it (cf. Summa Contra Gentiles lib. 2 cap. 68 on "how the intellectual substance can be the form of the body").
With identical twins, there are a few possibilities:
  • One zygote* could split into two, then God creates a soul for each of them (either simultaneously or asynchronously)
  • God creates a soul for one zygote,** and that zygote makes another cell to which God unites another soul.
*This single-celled zygote would be a situation where a zygote ≠ human
**or even a full-grown human being, as the technology for creating an ovum from an adult stem cell is under development, although it certainly isn't moral to fertilize such an ovum (as it separates procreation from the God-given rights of the potentially-conceived child to have a mother and father united in matrimony)


As St. Thomas Aquinas says in his Summa Contra Gentiles, lib. 2 cap. 87 tit., "the human soul is brought into being through the creative action of God," not through the biological processes of procreation, which are instrumental causes, but as its primary cause. In ibid., lib. 2 cap. 88 n. 3, Aquinas presents the argument of those who think the intellect arises from something genetic and not from an intellective or rational soul:
QuoteMoreover, as Aristotle [erroneously] teaches in the De generatione animalium [II, 3], the fetus is an animal before becoming a man. But, during the time in which the fetus is an animal and not a man, it has a sensitive [i.e., pertaining to the senses] and not an intellective soul; and, just as in other animals, this sensitive soul in indubitably produced by the active power of the semen. And yet that same sensitive soul is potentially intellective, just as that animal is potentially a rational animal; and the notion that the supervening intellective soul is substantially distinct from the sensitive one has been refuted already. It therefore seems that the substance of the intellective soul is derived from a power in the semen. [Viz., it is genetic.]
He countered this argument in Summa Contra Gentiles, lib. 2 cap. 86 n. 4-5 and ibid., lib. 2 cap. 89 n. 3, saying:
Quote[...] the intellective soul is the most perfect of souls and its power the highest [and] its proper perfectible subject is a body having many different organs through which its multifarious operations can be carried out; and that is why the soul cannot possibly be actually present in the semen separated from the body [...] The intellect, which is the proper and principal power of the intellective soul, is not the act of any part of the body, and therefore it cannot be divided accidentally as a result of the body's being divided [as through cell division]. Nor, then, can the intellective soul be so divided.
 Hence, from the hypothesis that the human soul is brought into being through the active power in the semen it follows that its being depends upon matter, as with other material forms. But the contrary of this has already been proved. The intellective soul, therefore, is in no way produced through the transmission of the semen.
 And the hypothesis of the soul's presence in the semen from the beginning would entail the further consequence that animal generation takes place solely by way of partition, as with annulose animals, where two are produced from one. For, if the semen were possessed of a soul at the moment of its separation, it would then already be endowed with a substantial form. But in every case substantial generation precedes the substantial form; it never comes after it; and if any changes follow in the wake of the substantial form, they concern not the being but the well-being of the thing generated. Thus, the engendering of the animal would be completed with the mere alienation of the semen; and all subsequent changes would have no bearing upon the process of generation.
 But this theory would be even more ridiculous if applied to the rational soul. For, first, the soul cannot possibly be divided as the body is, so as to be present in the separated semen; and second, it would follow that in all extra-copulative emissions of semen, without conception taking place, rational souls would nevertheless be multiplied.


See the chapter on embryology and humanness (attached below) in Fr. Benedict Ashley, O.P. (RIP 2013)'s The Ashley Reader: Redeeming Reason. It's an excellent overview, covering topics including spontaneous abortion and delayed hominization/ensoulment.

Also, Fr. Ripperger, FSSP's article on charitable surrogacy, which might also be of interest.