Thomas Aquinas 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 and that a substantial form directly informs prime matter 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 , arguing instead that the .. (shrink)