Recap: When does an embryo have a soul? - by Elissa McCormack

Saint Thomas Aquinas believed in what today is called “delayed hominization,” i.e., that the embryo has first a vegetative soul, then a sensitive soul, and it is only later that the embryo actually has a human soul. There are also two definitions of Thomas’s soul that we have to keep in mind: first, the soul is the substantial form of the body (so a human soul is what makes a human to be a human), and second, a soul could not be united to a body unless the potency of the body were disposed by its present form to be actualized by the human soul (there has to be a sufficient amount of organization to support the soul). However, Thomas’s theory is based on outdated biology, so we need to look at what his theory would look like when it is informed by a modern understanding of biology.

During the process of fertilization, the embryo receives its genetic structure, unique from that of the mother or the father, and then cell division begins. However, according to Thomas’s idea of the soul, we cannot say that the embryo has a human soul at this point because there are no structures in place to support the higher functions of the human soul, i.e., the intellect and the will. In fact, the brain is the last organ to develop and continues to develop until two years after the birth of the child.

Another problem with saying that the embryo has a soul at this point is the issue of twinning. Although with the genetic code the embryo is set to develop into a human being, at this early point in the pregnancy any cell that divides from the other cells has all the genetic material to develop into an entirely new human! Since an individual human soul cannot divide, the idea of an individual human being splitting into two human beings is kind of hard to take. Additionally, one-third to one-half of all fertilized eggs do not survive to implant. If the embryo is given a soul by God at the “moment” of conception, what happens to all these souls?

The interesting thing about implantation (ten to fourteen days after conception) is that this is the point when the possibility of twinning is removed and then the nervous system and the heart first begin to develop. I think that if we are using Saint Thomas’s idea of the soul, this is the point when God would infuse the embryo with a human soul. There is also no longer any chance of this “human” splitting into more than one human through twinning. Additionally, since the cells that make up the nervous system and the heart are beginning to differentiate themselves, this is also the first moment in the pregnancy that you can argue that the embryo has even the beginnings of the structures necessary to support the rational functions of the human soul.

What does this mean for the teaching of the Catholic Church? Not much actually. Since implantation is really the first moment that a woman can know that she is pregnant, the teaching on abortion would not change. Additionally the teaching on the morning-after pill would not change because, properly speaking, that pill is classified as emergency contraception and so would be covered under the Church’s teaching on contraception. The only teaching that might have to change (and I welcome comments on this issue because I am not as informed about it) is that on stem-cell research. The question as I see it is: what is the moral status of the artificially fertilized eggs that are never intended for implantation? If the human soul is connected to implantation, these eggs would not have moral human personhood.

However, since science cannot teach us morality, in this issue we need to turn to the certainty we get from faith and the teachings of the Church. Both John Paul II’s Evangelium Vitae and the Congregation for the Doctrine of the Faith’s Declaration on Procured Abortion teach that since we do not know when the embryo receives a human soul, it is better to respect the life of the embryo than it is to risk the sin of murder.