If, however, the Soul be one, why is some one soul reasonable, another
irrational, or some other one merely vegetative? The indivisible part
of the soul consists in reason, which is not divided in the bodies,
while the part of the divisible soul in the bodies (which, though being
one in herself, nevertheless divides herself in the bodies, because
she sheds sentiment everywhere), must be regarded as another power of
the soul (the sensitive power); likewise, the part which fashions and
produces the bodies is still another power (the vegetative power);
nevertheless, this plurality of powers does not destroy the unity of
the soul. For instance, in a grain of seed there are also several
powers; nevertheless this grain of seed is one, and from this unity is
born a multiplicity which forms a unity.