Thomas Aquinas and the Generation of the Embryo

Abstract

Thomas Aquinas is generally viewed as the chief proponent of the theory of delayed animation, the view that the human embryo does not at first have the rational soul proper to human beings. Thomas follows Aristotle's embryology, in which an embryo is animated by a succession of souls. The first is a nutritive soul, having the powers of growth, nutrition, and generation. The second is a sensitive soul, having the additional powers of locomotion and sensing. The third and final soul is the human, or rational soul, which virtually includes the nutritive and sensitive souls. Because Thomas holds that there is only one substantial form of a composite, none of these forms overlap to provide continuity. It is therefore exceedingly difficult to speak of the embryo as one enduring subject through the succession of souls. Moreover, because of the way that the nutritive soul is associated with plants, and the sensitive soul is associated with animals, interpreters generally hold that for Thomas the embryo is first a plant, then an animal, and with the advent of the rational soul, finally a human being. Those who write about the ontological status of the embryo assume that delayed animation necessarily entails delayed hominization, that is, that the embryo only becomes human at a later stage of its development, when it receives the rational soul. Those who hold a delayed animation view of the embryo often invoke Thomas' schedule of successive souls in the embryo as a model for viewing it as not yet human in early stages of development, linking hominization to the ability to perform intellectual operations. That Thomas specifies that a body must be sufficiently organized before the advent of the rational soul seems to them to solidify their view of the embryo as not sufficiently organized to be truly human. Additionally, even outside of an explicitly Thomist framework, Thomist metaphysical principles are often invoked in arguments that center on twinning and totipotency of blastomeres in the early embryo, and whether that early embryo is one individual if it is potentially many. Those who hold immediate animation views (i.e., the embryo receives the rational soul at once, with no mediate states) often adopt the strategy of importing modern data on the internal organization and self-directed development of the embryo, and argue that if only Thomas had known that the zygote was not unformed and undifferentiated, that it has within itself all it needs to become a mature adult human, he would have held that the embryo is immediately suited to receive the rational soul, and thus is human from conception. In this way they attempt to employ a change in scientific data to negate the need for a succession of forms in the embryo. The author identifies the being of the human embryo as a prior metaphysical problem within Thomas' work, and advances a different interpretation of his views: that the embryo, even before the advent of the rational soul, is human. To establish this claim, she traces the problems which emerge in the current debate about when the embryo becomes human, and argues that contrary to expectation, it is not necessary to equate immediate rational animation with immediate hominization, demonstrating that all other approaches yield results entirely untenable for Thomas. 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. Moreover, a distinction based on a passage in Aristotle's Generation of Animals with regard to an additional meaning of generation may resolve the ambivalence in Thomas' account of the embryo as passive under the formative power of the father's semen. Finally, a third meaning of generation is offered to show that Thomas recognized and wished to resolve the difficulty of explaining the continuity and identify of the embryo in the succession of souls. What results is an immediate hominization view of the embryo that, because it accommodates Thomas' succession of souls and does not depend upon importing modern biological data on the embryo, is consistent with Thomas' account, and is thoroughly cognizant of the way Thomas viewed human nature and the final end of human being.