Anthony Kenny reviews David Albert Jones' The Soul of the Embryo: An enquiry into the status of the human embryo in the Christian tradition and takes on the controversial question of when human life begins (via Verbum Ipsum). Kenny traces the evolving history of Christian thought on the topic, clearly demonstrating that "there is no such thing as the Christian consensus on the timing of the origin of the human individual."
The central issue, then, is to record, and decide between, the three alternatives from which we began: should we take individual human life as beginning at conception, at birth, or at some point in between? If the correct alternative is the third one, then we must ask further questions. When, in the course of pregnancy, is the crucial moment? Is it the point of formation (when the foetus has acquired distinct organs), or is it the point of quickening (when the movements of the foetus are perceptible to the mother)? Can we identify the moment by specifying a number of days from the beginning of pregnancy?
Many historical ideas of human development and "ensoulment" (when God gives the human individual a soul) sound quite foreign to our 21st century ears. Kenny touches on Christian thinkers who believed that the human soul existed before conception and the medieval idea of the homunculus (Latin for "little man"), the strange notion that the complete and very tiny human body existed in the father's semen before entering the mother's womb. The idea of human life beginning at conception was prevalent in the early church, including the writings of Tertullian and Gregory of Nyssa, but for most of Christian history has been a minority view. Thomas Aquinas, based on a misunderstanding of Aristotle, wrote of the embryo starting with a vegetative soul, progressing to an animal soul, and finally to a rational soul infused by God. Other pre-Reformation theologians believed that God infused the embryo with a soul when the form of the body was complete ("generally held to be 40 days or thereabouts" after conception).
With modern technology, today we know more than ever about the conception and prenatal development of human life. Because we are able to pinpoint the moment that a new genetically distinct individual is created, many Christians believe that this instant is when God gives the fertilized egg a human soul. Kenny bases his idea at a different stage, also illuminated by modern technology: he places the genesis of personhood after the possibility of "twinning," the point around 14 days after conception at which a single embryo divides into identical twins.
A cell that will become a human being - an embryo or conceptus - will do so within fourteen days. If it is not implanted within fourteen days it will never have a birth . . . . The basis for the fourteen-day limit was that it related to the stage of implementation which I have just described, and to the stage at which it is still uncertain whether an embryo will divide into one or more individuals, and thus up to the stage before true individual development has begun. Up to fourteen days that embryo could become one person, two people, or even more.
If the course of development of the embryo gives good reason to believe that before the fourteenth day it is not an individual human being, it gives equally good reason to believe that after that time it is an individual human being.
While in many ways, modern reproductive technology sheds new light on human development, it also muddies the theological picture considerably. What implications does our knowledge of "twinning" have on ensoulment? What about procedures like in vitro fertilization (IVF) and doctors' ability to freeze embryos created by IVF? (Does a human soul exist when an embryo is frozen?) What about the large percentage of fertilized eggs that naturally do not implant in the mother's uterus - do they have souls? We know so much more about the origins of individual human beings than Aquinas or other theologians ever did but no technology exists that can tell us any more about the human soul.