St. Thomas Aquinas was correct about many things, but he was wrong about abortion . . . at least indirectly.
This is not to say Aquinas was mistaken to suggest that abortion is seriously sinful, which he did. There Aquinas agreed with modern Church teaching. Where Aquinas erred was in setting up the potential inference that abortion up to a certain point does not constitute murder—in other words, that the child aborted in the first few weeks was not yet a human person, but matter, being gradually disposed toward the reception of a human soul at some later point.
It must first be noted that intended abortion is not something Aquinas explicitly discusses. (What we do get is the affirmation that someone has committed murder even if he unintentionally kills an animated—that is, sufficiently developed—fetus by striking the pregnant mother.) Rather, the mistake related to abortion is inferred by Aquinas’s belief that ensoulment occurs sometime after conception.
Who could have imagined pitting pro-life people against one of the greatest Catholic theologians of all time? But it’s not unheard of for people to resort to Thomas Aquinas—whom they would otherwise want little to do with, given his positions on everything from sexual promiscuity to homosexual activity—to justify the murder of babes.
To be fair, perhaps they do not think they are condoning the murder of babes, or maybe they are unsure, and just saying, “Hey, let’s not be arrogant about this. Look what Aquinas had to say. You wouldn’t think he’s crazy, do you?”
It is true that Aquinas believed in later ensoulment; thus, for him, abortion up to a certain point would not have constituted murder, though he would have maintained that it is morally impermissible—in fact, a grave offense. As the Congregation for the Doctrine of the Faith notes, “it is true that in the Middle Ages when the opinion was generally held that he spiritual soul was not present until after the first few weeks, a distinction was made in the evaluation of the sin and the gravity of penal sanctions . . . but it was never denied at that time that procured abortion, even during the first days, was an objectively grave fault. This condemnation was in fact unanimous.”
Following Aristotle, Aquinas thought ensoulment happened at about forty days’ gestation for a male and ninety days for a female. What led Aquinas to believe in later ensoulment were essentially four commitments, only one of which is metaphysical—to wit, that in the natural order, things are gradually brought from potential to act or imperfection to perfection. The remaining commitments are empirical, which John Haldane helpfully summarizes:
First . . . that the male is the sole active cause; second, the menstrual blood upon which the semen work has only a low degree of perfection or organization; third, as a consequence, the distance between the initial point (menstrual blood) and the end point (a body sufficiently organized to receive a human soul) is quite long.
As Haldane tells us, all three of these embryological points are now known to be false. This is critical to acknowledge. Aquinas’s metaphysical principle (that in the natural order, things are gradually brought from potency to act) may still be true—in fact, I think it is true—but, when applied to obviously false empirical assumptions, drives toward a faulty conclusion. No surprise there. Nevertheless, we should think Aquinas’s philosophy could be easily adjusted—in fact, that Aquinas would have adjusted his own stance—if he simply knew the mistakes of his empirical assessment.
For example, we know that the male is not the sole active cause in reproduction. On the contrary, what the female provides (a female gamete) is an extremely complex living cell, suffused with information and highly disposed for rapid embryonic development. It just needs the male seed. Thus, Aquinas’s principle concerning the gradual development of the simple to the complex can be applied to gametogenesis (the process by which precursor cells form into gametes). From fertilization on, the human being is fully present. So far, this is basic modern biology.
Now, Aquinas also claimed that the human soul could not be infused until certain material conditions (in this case, complexity) were met. This flows from Aquinas’s metaphysical notion that the material conditions must be proportioned to the form, which for living things is the soul. Hence, Aquinas thought the presence of organs was necessary, even if not fully developed. While we cannot examine this metaphysical notion (its validity or motivations) in detail, it suffices for our purposes to point out that Aquinas is simply laboring under another empirical mistake, since he was entirely unaware of the epigenetic “primordia” of the organs contained immediately at conception.
But Aquinas did not maintain that the material conditions had to be sufficiently developed to immediately exercise their capacities to identify the presence of a human soul. Rather, they just had to be there in some “primordial” state or beginning. Hence, Aquinas maintained that the rational soul was present at about forty days, even though he definitely did not think the brain was sufficiently developed to engage conceptual thought at that point. But now we know that the beginnings of the organs are there at conception.
So Aquinas’s metaphysical requirements are satisfied, and those holding the Thomistic position today should affirm the human person as fully present at fertilization. As Haldane tells us, Aquinas’s thinking should lead us to say, “What is necessary for ensoulment is the material organization sufficient for the development of those organs, in other words, the epigenetic primordia of the organs that support the operations proper to the species.”
We can thus retain Aquinas’s metaphysical commitments while correcting his false conclusion concerning abortion, simply by adjusting his empirical (embryological) mistakes. Aquinas did not have access to modern science; we do. So, whereas Aquinas may have had a reasonable excuse for thinking abortion failed to constitute murder in certain instances, those who seek to use him to justify abortion cannot appeal to such ignorance. The science on this matter is nowadays abundantly clear.