s like the generated, it follows of necessity that both the
sensitive soul, and all other like forms are naturally brought into
existence by certain corporeal agents that reduce the matter from
potentiality to act, through some corporeal power of which they are
possessed.

Now the more powerful an agent, the greater scope its action has: for
instance, the hotter a body, the greater the distance to which its
heat carries. Therefore bodies not endowed with life, which are the
lowest in the order of nature, generate their like, not through some
medium, but by themselves; thus fire by itself generates fire. But
living bodies, as being more powerful, act so as to generate their
like, both without and with a medium. Without a medium--in the work
of nutrition, in which flesh generates flesh: with a medium--in the
act of generation, because the semen of the animal or plant derives
a certain active force from the soul of the generator, just as the
instrument derives a certain motive power from the principal agent.
And as it matters not whether we say that something is moved by the
instrument or by the principal agent, so neither does it matter
whether we say that the soul of the generated is caused by the soul
of the generator, or by some seminal power derived therefrom.

Reply Obj. 1: The sensitive soul is not a perfect self-subsistent
substance. We have said enough (Q. 25, A. 3) on this point, nor need
we repeat it here.

Reply Obj. 2: The generating power begets not only by its own virtue
but by that of the whole soul, of which it is a power. Therefore the
generating power of a plant generates a plant, and that of an animal
begets an animal. For the more perfect the soul is, to so much a more
perfect effect is its generating power ordained.

Reply Obj. 3: This active force which is in the semen, and which is
derived from the soul of the generator, is, as it were, a certain
movement of this soul itself: nor is it the soul or a part of the
soul, save virtually; thus the form of a bed is not in the saw or the
axe, but a certain movement towards that form. Consequently there is
no need for this active force to have an actual organ; but it is
based on the (vital) spirit in the semen which is frothy, as is
attested by its whiteness. In which spirit, moreover, there is a
certain heat derived from the power of the heavenly bodies, by virtue
of which the inferior bodies also act towards the production of the
species as stated abo