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Was St. Thomas Aquinas Ever Wrong?

St. Thomas, a self-acknowledged limited and imperfect human being, did indeed make some mistakes . . . but we shouldn't hold it against him

Kevin Vost

St. Thomas Aquinas credited the pope with the authority to decide certain matters of faith with “unshakable certainty.” He never claimed as much for himself. Indeed, in the beginning of his Summa Contra Gentiles, Thomas apologizes to his readers, stating that in attempting the work, he might be exceeding his “limited powers,” but he embarks nonetheless with confidence “in the name of divine mercy.”

Thomas, a self-acknowledged limited and imperfect human being, did indeed make some mistakes—rarely in his philosophical or theological analyses, and most often when he illustrated principles with examples based on the limited scientific knowledge of the thirteenth century. For one example of the former case, in the realm of theology, it does not appear that Thomas fully anticipated the dogma of the Immaculate Conception, which was not promulgated as a dogma until 1854, nearly 600 years after his death.

In perhaps his most significant scientific error, Thomas has at times been cited as a supportive source by dissidents who defend the heinous practice of abortion. Let’s lay this out in the depth it deserves.

Thomas’s writings contain no endorsements of abortion and no mention of intended abortion. In fact, the unborn child is mentioned in only a few places in the Summa, in the context of other issues:

  • In II-II, Q. 64, a. 8, explicating Exodus 21:22, Thomas concludes that a man who strikes and kills a woman with child commits homicide whether the woman or the child dies.
  • In III, Q. 68, a. 11, buttressing his argument with Romans 3:8, he concludes that, because we may not do an evil in order that a good may result, if an unborn child is in danger of death, it is wrong to kill the mother so the child can be baptized, but if the mother dies, it is proper to open the womb to baptize the child.
  • In a beautiful passage in I, Q. 113, a. 5, citing St. Jerome, Thomas writes regarding the dignity of the unborn: “As long as the child is in the mother’s womb, it is not entirely separate, but by reason of a certain intimate tie, is still part of her: just as the fruit while hanging on the tree is part of the tree. And therefore it can be said with some degree of probability, that the angel who guards the mother guards the child while in the womb. But at its birth, when it becomes separate from the mother, an angel guardian is appointed to it.”

Clearly, these passages provide little material for pro-abortion posters or bumper stickers.

Abortion advocates also like to bring up Thomas’s writings on ensoulment—the point in development at which God infuses the human soul. Since Thomas believed that male souls are infused at forty days’ gestation and female souls at ninety, the logic goes, the Church must be comfortable with abortions committed before those points in human development.

Now, it’s true that Thomas did not hold to the Church’s later fully developed teaching that God makes present the fully human soul from the instant of conception. This is due largely to the limited knowledge of human embryology and the complete lack of knowledge of genetics in his time. With regard to biology, Thomas relied heavily on the writings of Aristotle. As rational and brilliant as both men were, not all of their arguments were based on sound scientific facts.

Nonetheless, Thomas knew well that a child must not be destroyed, even before ensoulment. He argued that the image of God is found in every human being, whether or not he has yet developed the inherently human powers of intellect and will (ST, I, Q. 93, a. 4). He argued as well that “it is quite clear that an intellectual substance is not united as a form to such a body except a human one” (Summa Contra Gentiles 2.90.2). Further, “every substance seeks the preservation of its own being, according to its nature; and by reason of this inclination, whatever is a means of preserving human life, and of warding off its obstacles, belongs to the natural law” (ST, I-II, Q. 94, a. 2).

We shouldn’t take the above to mean that Thomas was scientifically unsophisticated. On the contrary, his writings on prenatal development and ensoulment are complex, and he had the foresight to note—when using illustrations from astronomy, for example—that scientific theories accepted in his day could well be found wrong and changed in the future. Indeed, some modern Catholic thinkers argue that had Thomas had our current knowledge of human biology—for example, of the fact that the newly formed zygote has DNA comprising contributions by both parents at the instant of conception, distinct from the DNA of either of parent, he would have been a staunch defender of life from the instant of conception.

Moreover, in a recent doctoral dissertation on this issue, the author argues: “A survey of texts reveals that Thomas did in fact view the embryo as human before the rational soul, though he does not methodically work out the implications of that view in a number of areas.”

(Finally, don’t forget that the moral status of abortion was much better understood in Thomas’s day than it is, tragically, in ours. Perhaps a key reason he did not address the issue of abortion more directly in his writings is that the evil of abortion had always been a non-controversial tenet of Church teaching.)


This article is adapted from the 20 Answers booklet “Thomas Aquinas,” now available at the Catholic Answers shop.

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