One area where Aquinas’ theory has surprising implications is the development of the human embryo. Given that the Catholic Church is well known for insisting that a fully human life begins at conception, it’s intriguing to find that the Church’s most canonical thinker doesn’t think anything of the sort. Instead, Aquinas believes that the presence of a rational soul requires the presence of organs that can carry out its functions. Since this is lacking at early stages of fetal development, only the lower nutritive or plant-like functions are present at first.
