h the semen.

Reply Obj. 1: In the passage quoted, the part is put instead of the
whole, the soul for the whole man, by the figure of synecdoche.

Reply Obj. 2: Some say that the vital functions observed in the
embryo are not from its soul, but from the soul of the mother; or
from the formative power of the semen. Both of these explanations are
false; for vital functions such as feeling, nourishment, and growth
cannot be from an extrinsic principle. Consequently it must be said
that the soul is in the embryo; the nutritive soul from the
beginning, then the sensitive, lastly the intellectual soul.

Therefore some say that in addition to the vegetative soul which
existed first, another, namely the sensitive, soul supervenes; and in
addition to this, again another, namely the intellectual soul. Thus
there would be in man three souls of which one would be in
potentiality to another. This has been disproved above (Q. 76,
A. 3).

Therefore others say that the same soul which was at first merely
vegetative, afterwards through the action of the seminal power,
becomes a sensitive soul; and finally this same soul becomes
intellectual, not indeed through the active seminal power, but by
the power of a higher agent, namely God enlightening (the soul) from
without. For this reason the Philosopher says that the intellect
comes from without. But this will not hold. First, because no
substantial form is susceptible of more or less; but addition of
greater perfection constitutes another species, just as the addition
of unity constitutes another species of number. Now it is not
possible for the same identical form to belong to different species.
Secondly, because it would follow that the generation of an animal
would be a continuous movement, proceeding gradually from the
imperfect to the perfect, as happens in alteration. Thirdly, because
it would follow that the generation of a man or an animal is not
generation simply, because the subject thereof would be a being in
act. For if the vegetative soul is from the beginning in the matter
of offspring, and is subsequently gradually brought to perfection;
this will imply addition of further perfection without corruption of
the preceding perfection. And this is contrary to the nature of
generation properly so called. Fourthly, because either that which is
caused by the action of God is something subsistent: and thus it must
needs be essentially distinct from the pre-existing form, which