The Spirit of Life

Aquinas on the Beginning and End of Human Lifeby Fabrizio Amerini

Harvard University Press. 288p $29.95

In this erudite, thoughtful, carefully translated but sometimes turgid book, Fabrizio Amerini reviews Thomas Aquinas’s doctrine of the human fetus’s successive ensoulments, in order to “dialogue with the contemporary bioethical debate on abortion.” When, if ever, is a fetus developed enough to be regarded as a “human being” or “human person” with moral dignity, integral and perhaps inviolable rights? Any plausible answer to this question should use the empirical facts of human biology as well as the conceptual distinctions of philosophy, if we follow Aquinas’s example.


Aquinas rehashed, for the theological edification of his medieval Christian audience, Aristotle’s then 1,600-year-old embryology. Amerini exposes the “tensions” and “vacillations” in Aquinas’s abstruse accounts of human embryogenesis. But their main point is clear: the inseminating male initiates a process that, while it leads to the conception of a pre-human uterine animal, does not actually generate another human being.

According to Aquinas, embryogenesis starts with the male’s ejaculated sperm which, though not ensouled, is endowed with a “formative power” that mysteriously projects the psychic force of the inseminating male. The formative power moves the sperm’s “vital spirit” which, in turn, thermodynamically shapes the fetus’s primary organs. In the uterus, the sperm encounters living sanguineous matter animated by a vegetative soul, a form present in “first act.” The seminal vital spirit prompts the quiescent vegetative soul into operation or “second act”: the vegetative embryo takes in nourishment, grows and begins to manifest primitive organs. After it draws forth the sensitive soul from the organically developing vegetative matter, the sperm dissolves and the sensitive soul subsumes directing nutrition and growth, and completes the emergent organs.

Thus far, no fetal human being has yet appeared in the womb. Nor could one ever appear without supernatural or divine help. The specifically human or rational soul is an immaterial (spiritual) form that cannot be physically transmitted or causally educed from the maternal matter by the paternal sperm. To the philosophically naïve observer, it looks as though there is but a single same embryo that physically morphs during gestation into a recognizable human fetus. But rationally analyzed, the “process of becoming a human being” involves a series of substantial generations and corruptions, a temporal succession of vegetative, sensitive and rational souls. This succession of different souls metaphysically jeopardizes the numerical identity of the embryo/fetus passing from one living species to another.

Aquinas’s solution, says Amerini, is merely a “simple stipulation” that the same single embryo “naturally and of itself becomes a human being”; the embryo is in potency what the human being is in act. This is a commonsensical but surprising stipulation. Logically, Aquinas’s embryology leads to a different and more paradoxical conclusion: Whatever the lead-up, the actual conception of a human being is not a natural event. Because the pre-human fetus has no natural active potency to become a rational animal, God himself must create and supernaturally infuse the rational soul ab extra.

Of course, the continuous human identity of the developing fetus would be evident if God supernaturally infused a single, rational soul-form at conception. But the received or Aristotelian definition is that the “rational soul” is “the act of an organic body with potential life,” which refers not to an inanimate body’s potentiality for coming alive, but to a fetus’s potentiality for rational life. That potentiality requires that the fetus be sufficiently “organ-ized” to receive an infused rational soul. When does fetal organization reach that point?

Aquinas follows Aristotle’s embryological timetable, which is based on post-factum appearances. The fetus can be judged rationally ensouled when it is seen to have the requisite organs for intellectual activity—an incipient brain. That happens for the embryonic male by the 40th or, as Augustine thought, the 46th day, but for the defective female fetus only on the 90th day. Rational ensoulment occurs, Amerini explains, when the fetus’s “organic body has in act the capacity to be able to exercise intellectual functions, even if in fact it does not exercise them nor is able yet to exercise them.” Disambiguated, this means that the fetus must be “organ-ized” enough to sustain a soul in ‘first act’ whose proper accidents are its intellectual powers, but need not be organically so advanced as to enable immediately the soul’s “second act,” the actual exercise of its intellectual powers.

Now what do we relevantly know that Aquinas did not about the “organ-ized” fetal body? We know that human conception is the fusion of two haploid cells, the male sperm and the female ovum, each with 23 chromosomes. The resulting diploid cell, the genetically unique zygote with 46 chromosomes joined in 23 pairs, contains all the genetic material and instructions (genes each made up of two intertwined strands of DNA) necessary for the full development of a complete organism. Aristotle and Aquinas could only look for and see a tiny brain, from which they inferred the rational humanity of the fetus. Decades of X-ray crystallography and, more recently, the far sharper images taken by electron microscopy have given us much better spectacles. In 2012, a team led by Enzo di Fabrizio directly imaged a six molecule cord of the DNA helix. Upon this kind of empirical data rests a strong if not dispositive Thomist philosophical argument that the rational soul informs the zygote or original diploid cell.

Aquinas reiterated the canonical teaching of his day: aborting a rationally “unformed fetus” is a grave sin but not morally equivalent to homicide. Amerini, however, argues that Aquinas cannot be pressed into the service of the contemporary Roman Catholic ecclesiastical magisterium: Prior to the infusion of the rational soul—when the fetus is only a potential human being—Aquinas’s embryology, “is fully compatible both with a position in favor of and against abortion.” No less provocatively, Amerini also claims that “any level and type of scientific knowledge that one may have . . . [about] the moment of conception” is irrelevant to judging the soundness of Aquinas’s views on when the fetus in the womb can be considered to be a ‘human being.’

Yet the question remains: If a tiny, 40-day-old fetal brain is a sign of a fetal body adequately disposed for rational ensoulment, why is not the DNA of a zygote an even more obvious sign from “Day One”? The tiny brain seen by Aristotle is already present at the moment of conception, formally not materially, in the zygote; the form of the organs necessary for rational humanity is instantiated in the structured gene sequences found in the diploid cell’s DNA. Given the formal (DNA) structure of the zygote, the subsequent development of a brain organ small or big is, from an Aristotelian standpoint, an accidental and not a substantial change. Admittedly, one should interpret Aquinas’s own texts in sensu stricto, but that hermeneutical requirement hardly precludes applying Aquinas’s philosophical principles to new empirical data.

Something other than Aquinas is holding sway over Amerini’s exegesis. It is his conviction that Aquinas’s attribution of rational humanity to the 40-day-old male fetus is a “philosophical decision,” a rationally justified “stipulation” “independent of biology,” a choice assigning significance to man’s essential properties.

Sed contra: Aquinas does not explain human conception in terms of the alleged split between the empirical data of a scientific embryology and the universal Aristotelian philosophical principles (not, by the way, of metaphysics but physics) used to understand these data. In the Summa Contra Gentiles (II, c. 83; ed. Pera, 2: 243a, §1674b), Aquinas states that sensation and memory provide the experience [experimentum] that leads to “an understanding of the universal principles of the sciences and arts.” Whatever their empirical lacunae and aprioristic tendencies may be, Aristotelian natural sciences are normatively based “on reason and sensible experiments,” and it is the latter “that more makes for assurance [fides] in natural things” (III Sent., 3, q. 5, a. 1, resp.; ed. Moos, 142, §193). Would Aquinas have agreed with Amerini that empirical evidence is “totally nonessential for determining the criteria for what is meant by ‘human life’? To this contentious assertion Thomistic scholars can, should and likely will raise formidable textual and logical objections.

The church has good biblical and theological reasons for sacralizing sex and human conception. But she has not defined and perhaps never will define, as a dogma to be held on supernatural faith, that the human person begins at conception. Contemporary embryology presupposes what modern molecular biology has demonstrated—with a conceptual clarity and empirical detail that would have astonished Aristotle and Aquinas—about the human genome. Philosophy has to take up the task of showing that the zygote with a human genome is a developing “human person.” From the standpoint of a natural ethics, the church’s rejection of even the earliest post-conception abortion remains a prudential judgment based on reason, generous love and the deepest respect for everyone “human.” About what is human, prudential moral reasoning has much to learn from contemporary biology.

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