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Potentially Human? Aquinas on Aristotle on Human Generation

Thomas Aquinas (1225–74) describes embryological development as a succession of vital principles, souls, or substantial forms of which the last places the developing being in its own species. In the case of human beings this form is the rational soul. Aquinas' well-known commitment to the view that there is only one substantial form for each composite (unicity thesis [UT]) and that a substantial form directly informs prime matter (diPM) leads to the conclusion that the succession of soul kinds is non-cumulative. The problem is that this view seems to entail discontinuity in the process of generation. Aquinas argues for the continuity of a conceived being by appealing to the teleological argument at the core of Aristotle's embryogenesis theory: according to Aristotle, embryonic development is a gradual actualization of the form of the species, potentially present in the embryo from the outset. Aquinas denies such a gradualist account due to his metaphysical commitments (UT, diPM), arguing instead that the embryo prior to the reception of the rational soul is ‘potentially human’ only in the sense that the goal achieved by the process is to belong to the same species as the parents. In other words, a being conceived by human parents is potentially human because it is to become human at the terminus of a process, if nothing hinders its development. I argue that this justification fails because Aquinas, as with most authors of his time, also accepted the principle of the divine origin of the rational soul; applying this principle would mean that the end of the process is beyond the embryo's potentiality. Therefore I claim that the embryo is potentially human in a weak sense of the term: its aim is to develop a body appropriate to receiving the rational soul.