People are sometimes surprised to hear that the wrongness of destroying a human embryo does not ultimately depend on when that embryo might become a person, or when he or she might receive a soul from God. They often suppose that the Catholic Church teaches that destroying human embryos is unacceptable because such embryos are persons (or are "ensouled"). Yet the magisterium of the Church has never definitively stated when the ensoulment of the human embryo takes place. It remains an open question. The Declaration on Procured Abortion from the Congregation for the Doctrine of the Faith in 1974 phrases the matter with considerable precision:

This declaration expressly leaves aside the question of the moment when the spiritual soul is infused. There is not a unanimous tradition on this point and authors are as yet in disagreement. For some it dates from the first instant; for others it could not at least precede nidation [implantation in the uterus]. It is not within the competence of science to decide between these views, because the existence of an immortal soul is not a question in its field. It is a philosophical problem from which our moral affirmation remains independent…

That being said, the moral teaching of the Church is that the human embryo must be unconditionally protected and treated as if it were already ensouled, even if it might not yet be so. It must be treated as if it were a person from the moment of conception, even if there exists the theoretical possibility that it might not yet be so. Why this rather subtle, nuanced position, instead of simply declaring outright that embryos are ensouled, and therefore are persons? First, because there has never been a unanimous tradition on this point; and second, because the precise timing of ensoulment/personhood of the human embryo is irrelevant to the question of whether we may ever destroy such embryos for research or other purposes.

Interestingly, ensoulment has been discussed for centuries, and so-called delayed ensoulment was probably the norm for most of Christian history, with immediate ensoulment gaining some serious momentum of its own beginning in the 1600s (and probably representing the position most widely held today). Augustine seemed to shift his opinion back and forth during his lifetime between immediate and delayed ensoulment. In the 1200s, Thomas Aquinas held that human ensoulment occurred not right at the first instant, but at a time-point removed from the beginning. This, he argued, would enable the matter of the embryo to undergo development and become "apt" for the reception of an immortal soul from God (by passing through simpler initial stages involving “vegetative” and “animative” souls). Even today in various quarters, the discussions continue, with new embryological details like twinning and chimerization impinging upon the debate, and new conceptual questions arising from the intricate biology surrounding totipotency and pluripotency.

We must recognize that it is God's business as to precisely when He ensouls embryos. We do not need an answer to this fascinating and speculative theological question, like counting angels on the head of a pin, in order to grasp the fundamental truth that human embryos are inviolable and deserving of unconditional respect at every stage of their existence. Rather, we only need to grasp the key insight that every person on the planet is, so to speak, an “overgrown embryo.” Hence, it is not necessary to know exactly when God ensouls the embryo, because, as I sometimes point out in half-jest, even if it were true that an embryo did not receive her soul until she graduated from law school, that would not make it OK to kill her by forcibly extracting tissues or organs prior to graduation.

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 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.

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