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Aquinas on When Human Life Begins

If, as now seems likely, Roe and Casey are overturned by the Supreme Court, public debates will begin in dozens of states about when human life begins. There’s much confusion on the subject, even among Catholics. So, it’s imperative that we look to reliable guides for wisdom on this crucial matter.

Unreliable guides are everywhere. Consider, for instance, President Biden’s recent assertion:

Roe says what all basic mainstream religions have historically concluded, that the existence of a human life and being [sic] is a question. . . .Is it at the moment of conception? Is it six months? Is it six weeks? Is it quickening, like Aquinas argued?

In a misguided effort to make abortion permissible in the early stages of development, Biden cites Aquinas’s use of the medieval theory of “quickening” to suggest that even the Catholic Church isn’t sure if the embryo is a human being from conception onward.

Biden’s claim must be challenged. And so must its “twin” contention from Roe: if medical, scientific, and religious experts can’t agree when, in its developmental journey, the embryo/fetus becomes a human being, then we cannot declare if or when abortion destroys a human life. A pregnant woman, therefore, should be legally free to abort her “pregnancy” – a mere clump of cells.

But this is just another example of a politician spreading misinformation while quashing truth in the public square. The question of when human life begins is critically significant not only for the preborn but for every last one of us. As a simple matter of justice, then, we must answer this question truthfully.

How, then, do contemporary Thomist scholars reconcile Aquinas’s use of medieval biology with Church teaching on abortion? The issue is this: on the one hand, Aquinas, writing in the 13th century, described delayed hominization of the newly conceived human embryo – relying on Aristotelian and medieval biology. On the other hand, Aquinas would argue today – in light of indisputable 21st-century embryology – that the human embryo is a human being immediately from its single-cell stage forward.

But we need to explore this subject in greater detail.

Rejecting the Pythagorean and Platonist notion that the body is the “tomb” or “prison” of the soul, and that the soul is infused into a body that is alien to it, Aristotle saw the soul as the natural form of the body. The soul not only organizes, unifies, and specifies the matter of the human body, it’s also never opposed, or foreign, to the body.

Aristotle (and Aquinas after him), however, did not believe the human soul – with its complex spiritual powers – could be present at fertilization because the “matter” of the pregnancy was not proportionate to the “substantial form” of the human soul.

It’s important to understand what Aristotle and Aquinas meant by the concept of “sufficient matter” – the idea that matter or the material principle of a pregnancy must be proportionate to a rational soul.


Both philosophers held that, at conception, the only available material substance was the woman’s menstrual blood, a homogenous mass without any form or structure of its own. The matter of the menstrual blood had to be informed by the external agent of the semen that remained in the womb post-conception and that formed the menses in a series of progressively perfecting phases.

This formative process took approximately forty days for males and eighty days for females whereby the semen formed the menses first to the level of physiological (vegetative) life and then to the level of sentient (animal) life. Aristotle argued that only when the fetal body reached this higher state of formation did it receive its final organization by a human form, a rational soul.

Based on this – and on a medieval biology that differed even from Aristotelian biology – Aquinas concluded that, in all probability, God did not unite the body and soul until later in embryonic development.

Once we understand the Aristotelian/Thomistic (A/T) theory of delayed hominization, however, four facets of President Biden’s misappropriation of that theory surface.

1. Although the A/T theory was based on inaccurate biology, there is truth in its underlying principle, namely, that the soul requires sufficiently complex matter. This truth, combined with contemporary biology on the DNA at conception, warrants a true conclusion: the being or the “matter” of the zygote and early embryo is human because its body is human. That is, a greater part of the information needed to construct the zygote’s embryonic, fetal, and adult human body, including the human brain, is contained in its nuclear DNA. It’s a human body because it’s brought to life – informed, organized, unified – by the life principle of a human intellectual soul. In sum, the single-celled human zygote – a human body formed by a human soul – is a human being, i.e., an individuated human organism.

2. While fetal events like the appearance of the primitive streak, the developed brain, or “quickening” demarcate important stages in the maturation of the individuated human organism that began at fertilization, they do not signal the emergence of a new organism where there was previously none.

3. Contemporary theologians, bioethicists, and politicians (even Catholics like Biden) who invoke the A/T delayed hominization theory to postpone humanhood to some point post-conception fail to grasp the theory’s primary principle: a particular body is human when it is animated or informed by a human soul.

4. It is a paradox wrapped in an enigma why a Catholic president would cite, without accurate appropriation, this obsolete controversy for delayed hominization.

In sum, public discussion of when human life begins ought to include the most current embryology as interpreted by sound philosophical anthropology. It should neither revert back to medieval science nor misappropriate Aquinas’s use of that biology to defend the mistaken theory of delayed hominization.

Such uses of Aquinas need to be seen for what they are:  partisan politics not authentic science or religious principle.


*Image: The Immaculate Conception [1] by Francisco Rizi, 17th century [Museo del Prado, Mardrid]

You may also enjoy:

Hadley Arkes’ The Constitution and the March for Life [2]

Randall Smith’s From My Mother’s Womb, You Are My Strength [3]

Sister Renée Mirkes, OSF, Ph.D., a new contributor, is a Franciscan Sister of Christian Charity and the director of the Center for NaProEthics, the ethics division of the Saint Paul VI Institute, Omaha, NE.