Aquinas on Abortion


The Christian Church was against  against abortion from the earliest days (see The Fathers on Abortion). It can be something of an embarrassment, then, for Christians to discover that the views of the Church’s greatest theologian, Thomas Aquinas, have been used to support abortion.

The specific view in question has to do with Aquinas’s statement that God created the rational human soul in the body sometime after conception: namely, 40 days for males and 80 or 90 days for females (Commentary on the Book of Sentences, Bk. III, d.2, q.5, a.2, resp). Thus, it is argued, early abortion for Aquinas would not be murder, because what was being killed was not yet human.

Nor is this just some urban legendsustained in its existence by e-mail chains or popular blogs. This view of Aquinas was even cited in Roe v. Wade:

“It is undisputed that at common law, abortion performed before “quickening” – the first recognizable movement of the fetus in utero, appearing usually from the 16th to the 18th week of pregnancy 20 – was not an indictable offense. . . . [which] appears to have developed from a confluence of earlier philosophical, theological, and civil and canon law concepts of when life begins. . . . Christian theology and the canon law came to fix the point of animation at 40 days for a male and 80 days for a female, a view that persisted until the 19th century, there was otherwise little agreement about the precise time of formation or animation. . . . Due to continued uncertainty about the precise time when animation occurred, to the lack of any empirical basis for the 40-80-day view, and perhaps to Aquinas’ definition of movement as one of the two first principles of life” (ROE v. WADE, 410 U.S. 113 (1973): IV.3)

But would Aquinas agree that this modern position is a legitimate conclusion from his beliefs? I will argue that he most certainly would not.

Aquinas or Aristotle?

The first thing to realize is that that Aquinas wrote prior to the age of modern scientific methodology (he preceded even Roger Bacon) and technology. Natural science at the time was often more akin to what is more narrowly defined as philosophy today. Empirical data was lacking, and so the timing of “ensoulment” (the coming to be of the rational soul in a body) was based on simple observations. When Aquinas stated that the rational soul came to be in the body at 40 or 80/90 days, he was merely following Aristotle (History of Animals Bk. 7, pt. 3).

Now, conclusions follow from principles joined to particulars, for example: “All men are mortal” (principle), “Socrates is a man” (particular), therefore “Socrates is mortal” (conclusion). Both principle and particular must be true for the conclusion to follow as necessarily true. Thus, to conclude based on Aquinas’s mistaken trust of Aristotle’s particular position on the timing of ensoulment that he would agree with the early abortion advocates’ conclusion would fail without showing that it would follow from Aquinas’s principles and the confirmed particulars of modern science.

Philosophy or Science?

Aquinas got his “particulars” from the common science of his day. He applied his philosophical principles to the particulars supplied by authorities in other disciplines. Mistakes in Aquinas’s conclusions (or illustrations) are often traceable to the mistakes of the “scientists” he relied upon for particular facts about nature etc. The combination is sometimes difficult to distinguish, but it is important here – for if Aquinas were supplied with today’s accurate scientific particulars then it is probable that he would not believe in post-conception ensoulment.

In brief, Aquinas taught that male sperm combined with female (menstrual) blood and acted upon it as a formative power that produced first a body with a “vegetative soul” (i.e., one that was alive but not sentient). Then, once the material was organized to a sufficient degree the vegetative soul was replaced by an “animal soul” (i.e., one that was sentient but not rational). Once this body had become sufficiently organized it could then receive a “rational soul.” It was at this point that God would specially create what we today more commonly refer to as a “soul” (i.e., a human soul) for the body and the fetus would be fully human. Again, this was said to occur around day 40 for males and 80 or 90 for females.

Today, however, we know that the sperm is not just an organizing power – it makes up the material cause of the body as well. Further, menstrual blood has nothing to do with the process, but rather the ovum – which is also a material cause of the body. Once the two are joined both cease to exist, however, with no remainder. Thus, on Aquinas’s account whatever life-giving principle grows this new unity into a full-grown human body must be present immediately.

Now, there is a lot more detail to Aquinas’s position (see SCG 2.88, and Haldane and Lee) – but in reality the more detailed one gets, the more problems for a post-conception ensoulment become. Thus, although Aquinas’s philosophy might exclude the traducian view of conception, his principles would, in light of modern scientific knowledge, seem to lead him to view conception and ensoulment as twin features of the single event of human creation. Thus, any abortion would indeed be murder.

Aquinas on Abortion

Aquinas never really deals with abortion per se, but he does have a few things to say that bear upon it implicitly.

In his commentary on murder (ST IIae, q.64, a.8) he cites Exodus 21:22 concerning the killing of a pregnant woman and concludes that, “the death either of the woman or of the animated fetus, he will not be excused from homicide.”

Elsewhere,  Aquinas writes concerning whether or not a woman should be “opened up” to permit the baptizing of a pre-born baby whose life is in danger that it should not be done because murder is not excusable to save another life (ST III, q.68, a.11). The point for this discussion is that the infant was indeed considered to be alive and, virtually, able to be baptized.

NOTE: At the very least the act of abortion would remain a grave sin even if it was not seen as the murder of a human being. Following from the Church’s view that the materials of  human life garnered respect as such (and hence, the Church’s prohibitions against masturbation and contraception), an abortion would still be destroying that which was to become a rational animal (cf. Augustine on Abortion and Ensoulment).


Despite what some may think could be done with Aquinas’s position on the timing of ensoulment, given Aquinas’s principles it seems clear that Aquinas would not conclude the same things given today’s science. In any case, however, was in line with the Church on this issue – and thus it would be illicit to ascribe any kind of pro-abortion stance to him.

As evidence of this, once science corrected Aristotle’s erroneous thoughts on ensoulment, the Church was quick to apply its theology and philosophy to this new knowledge. Haldane and Lee report that as early as 1879 papers were being published arguing for a coincidence of conception and ensoulment (e.g., ‘De animatione foetus’ in Nouvelle Revue Theologique), and as these views took hold in science and philosophy they were adopted. By the end of the 19th Century the Roman Catholic Church (who considers Aquinas to be the supreme theologian) declared that  excommunication was required for abortion at any stage of pregnancy. (Unfortunately, while early Protestant church leaders such as Martin Luther and John Calvin were anti-abortion, but many Protestant denominations followed the secular drift in the 1960’s and have not found their way back. Evangelicals seem to have come around in the late 1970’s.)

Thus, based on application of Aquinas’s principles to modern knowledge of conception’s particulars, his actual statements that touch on the issue of abortion, and the reaction of the Church to the modern scientific understanding of conception, it is safe to conclude that Aquinas cannot legitimately be used to bolster a pro-abortion stance.