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A point that bears on moral issues in our own times has to do with the timing
of the arrival of
the rational soul in fetal development. Grosseteste had main-
tained that the rational soul is infused at conception but uses only its lower veg-
etative and sensitive powers until the body develops. Instead of this, Aquinas
claimed that since the generation of one thing necessarily entails the corruption
of another, “when a more perfect form arrives, the prior form is corrupted; pro-
vided, however, that the succeeding form” can perform all of the functions of the
preceding forms. The rational soul can
do this. So, in Aquinas’s view, prior to
the arrival of the rational soul, the growth and organization of the embryo is
directed first by the vegetative soul and then subsequently by the sensitive soul,
upon whose arrival the vegetative soul is obliterated. Both of these souls are bio-
logically transmitted. The rational soul, by contrast, “is created by God at the end
of human generation .” When the rational soul arrives on the scene, it is “at once
both sensitive and vegetative, the preexisting forms having been corrupted.”
10
What this means is, first, that the rational soul arrives relatively late in the
process of the development of the human embryo and that before its arrival, the
developing proto-embryo has no human soul, hence no soul of any kind that is
capable of surviving bodily death. In other words, in Aquinas’s view, what we
call the conceptus, that is, the fertilized egg that eventually will develop into an
embryo, is not, either at the moment of conception or for quite awhile afterward,
endowed with an immortal soul. In fact, technically speaking, it is not even
human. All of that happens later.
So far as survival of bodily death is concerned, in Aquinas’s view certain pow-
ers, such as those belonging to the sensitive and vegetative parts, “are in the com-
posite as their subject” and, hence, perish with the body. Other powers, such as
the intellect and the will, inhere only in the soul and thus “must remain in the
soul, after the destruction of the body.”
11
Nevertheless, what has sensations is nei-
ther the soul alone nor the body alone nor, as Augustine and other Neoplatonists
thought, the soul using the body, but the human being as a whole; soul and body
each play a part in producing sensations, which belong to both in union rather
than to either separately.
Some forms are capable of existing independently of matter and some not.
The ones that are capable are spiritual, or intelligible, substances. Some of these,
such as angels, are complete in that they are purely intelligible and have no func-
tions or activities that require material bodies. What makes them substances is
that they are a combination of form and existence. Others, such as human ratio-
nal souls, are incomplete in that they are not purely intelligible and have func-
tions and activities that require material bodies. But they too are a combination
of form and existence. In other words, Aquinas, unlike Bonaventure but quite