Conjoined Twins, Medieval Biology, and Evolving Reflection on Individual Identity

Scholastic discussions concerning conjoined twins turned to issues unarticulated by early medieval authors and brought into sharp focus the biological causes of these anomalous births based on Aristotelian natural philosophy. For Albert the Great, irregular or excessive movement during intercourse and a ruptured membrane in the womb may bring about either a complete division of the sperm, resulting in separated twins, or a partial division, resulting in conjoined fetuses. Following Albert, philosophers and theologians reflected upon theories of personhood when they concluded that conjoined twins with two heads may be two persons with two rational souls, and therefore should be baptized separately or even may contract separate marriages. This determination demanded consideration of empirical signs of personhood and focused attention on a Galenic-Aristotelian debate between “physicians” and “philosophers” to assert the heart’s priority over the head as the body’s principal organ and as the essential “seat” for the rational soul.