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  • The Role of the Virtus Formativa In St. Thomas Aquinas's Account of Embryogenesis
  • Carl A. Vater

DELAYED HOMINIZATION, as a way to defend the legitimacy of at least early-term abortions, has been defended in three ways. The first way, the fitting-matter argument, attempts to defend the legitimacy of a succession of forms in generation. According to this argument, the matter of the fetus is not fit to receive the rational soul (and so is not human) until it is sufficiently complex. Until the fetus becomes human by the reception of the rational soul, abortion is not morally different from killing a vegetable or an animal. The second way, the twinning argument, argues that because the embryo can twin in the first fourteen days of development, the embryo is not yet an individual substance and so cannot be accorded the moral status granted to all persons. The third way, the internal organization argument, argues that the embryo lacks the cellular differentiation to be a single, organized substance.1

All three arguments focus on the classical definition of the person as an individual substance of a rational nature, and they find the embryo lacking. The fitting-matter argument finds the embryo lacking a rational nature. The twinning argument finds [End Page 113] the embryo lacking individuality because one embryo can become two embryos. The internal organization argument finds the embryo lacking individuality because its cells seem to operate independently of each other.

This article will focus exclusively on the fitting-matter argument. Defenders of this argument often root their claim in the thought of St. Thomas Aquinas, saying that his account of embryogenesis provides the philosophical basis for defending their claims. They maintain that the philosophical principles he uses are valid despite his faulty biology and that Thomas would endorse their position.2 Rooting their arguments in his teachings is possible, they say, because the philosophical principles do not depend on the false biological data.

This article challenges the claim that Thomas's false biological claims can be separated from his philosophical claims. In what follows I will attempt to show that holding a truly Thomistic theory of fitting-matter delayed hominization demands accepting aspects of Thomas's biology as well, particularly the formative power (virtus formativa). To accomplish this goal, I will first give a brief outline of the fitting-matter argument for delayed hominization. Second, I will outline Thomas's account of embryonic development. Third, I will examine his uses of virtus formativa to determine its exact role and function in his account of embryogenesis. Finally, I will show that the virtus formativa plays an inextricable role in Thomas's account of embryogenesis. [End Page 114]

I. The Fitting-Matter Argument For Delayed Hominization

It is indisputable that Thomas argues that the rational soul is not given to an embryo at conception and so teaches that the embryo is not legitimately human until some point during the pregnancy.3 It must also be admitted that he had no access to contemporary biological data and so also teaches that the semen becomes blood after conception.4 On the face of it, the connection between these two teachings is unclear. While some have argued that if Thomas had access to contemporary biology he would have abandoned his delayed hominization, others have claimed that this philosophical principle of delayed hominization remains valid today in spite of the faulty biology that Thomas unites with it.5 Hylomorphism, it is argued, demands subscription to a system of delayed hominization because the rational soul, which is the substantial form of the body, cannot be joined to the fetus's body until the body has become fit to receive a rational soul.6 The body has to be fit to receive the rational soul because, as Aristotle says, "the soul is the act of a physical organic body having life potentially."7 God could not put a human soul in a rock, or a tulip, or even a kitten, because their bodies are not properly disposed to [End Page 115] intellection.8 Thus, these scholars hold with Thomas that the fetus undergoes a succession of generations and...


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