Aquinas on Human Soul and the Beginning of Life

There are important metaphysical reasons that induce Aquinas to think that the human embryo becomes a human being not immediately, at conception, but after some time of gestation. As I illustrated in my book "Aquinas on the Beginning and End of Human Life" (Harvard 2013), the principal one is that Thomas takes seriously Aristotle’s characterization of the human soul as the actuality of an organic physical body having life potentially. For him, the human soul is the substantial form of the human body, what makes a body, which has the potentiality to live, actually living. This means various things, but first that the human soul plays a key role in establishing the borders of the sphere of the human. The embryo can be called human precisely when it gets ensouled humanly, neither before the human soul has informed the body, nor after the human soul has departed it. The characterization of the human soul as the substantial form of the body has also a second implication, namely that the human soul can enter into a body only when the body has reached an appropriate material organization. This interpretation seems indisputable given Thomas’s pronouncements, but some scholars have nonetheless expressed worries about it and about, more basically, the scientific reliability of Thomas’s embryological explanation. In a recent review article of my book, for example, Patrick Toner faulted me for not criticizing Thomas’s embryological teaching in the light of today’s science and, as a consequence, for not replacing Thomas’s old-fashioned account of human embryogenesis with a new, but equally based on Thomas’s texts, explanation. The purpose of the present article is to go through Thomas’s embryology once again, for establishing what can be accepted or rejected of Toner’s criticism. My final conclusion is that, pace Toner, there is not enough evidence in Thomas’s works to attribute to him the thesis of the immediate hominization of the human embryo.