Acquiring a soul
Aristotle’s arguments, that the formation of a new being was gradual and the soul acquired some time after conception, informed medieval ideas.
The ancient Greek philosopher Aristotle argued that the female provided the menstrual blood as the passive material from which the active male semen generated the new form. While observing chick eggs at different stages, he became convinced that the form gradually emerged from unformed matter in a process he named ‘epigenesis’. Epigenesis required a soul. All organisms had a vegetative soul, and animals also a locomotor one, but the rational soul was reserved for humans. Aristotle drew on humoralism, which postulated that physical and mental features are determined by an innate balance of qualities such as heat, coldness, moisture and dryness. The hotter male embryos were ensouled around the fortieth day of pregnancy while the cooler females took twice as long.
For Christians, the soul had a different meaning and purpose. Some theologians placed ensoulment, or the acquisition of a God-given immortal soul, at conception. Yet from the late Middle Ages the Aristotelian view dominated. For practical purposes, quickening tended to be interpreted as coinciding with the entry of the soul. Understanding the early embryo as not-yet-human contributed to widespread tolerance of abortion. This would begin to change only in the Enlightenment