This paper examines some of the metaphysical assumptions behind Aquinas’s denials that a human rational soul unites with matter at conception and that a human rational soul is capable of developing and arranging the organic parts of an embryo. The paper argues that Buridan does not share these assumptions and holds that a soul is capable of developing and arranging organic parts. It argues that, given hylomorphism about the nature of organisms, including human beings, Buridan’s view is philosophically superior to (...) Aquinas’s in several respects. Finally, the paper poses an apparent inconsistency between several of Buridan’s texts on this topic and attempts to show that the inconsistency is merely apparent. (shrink)