Now every use implies movement, taking movement
broadly, so as to call thinking and willing movements, as is clear
from the Philosopher (De Anima iii, 4). Now in corporeal things we
see that for movement there is required not merely the form which is
the principle of the movement or action, but there is also required
the motion of the first mover. Now the first mover in the order of
corporeal things is the heavenly body. Hence no matter how perfectly
fire has heat, it would not bring about alteration, except by the
motion of the heavenly body. But it is clear that as all corporeal
movements are reduced to the motion of the heavenly body as to the
first corporeal mover, so all movements, both corporeal and
spiritual, are reduced to the simple First Mover, Who is God. And
hence no matter how perfect a corporeal or spiritual nature is
supposed to be, it cannot proceed to its act unless it be moved by
God; but this motion is according to the plan of His providence, and
not by necessity of nature, as the motion of the heavenly body. Now
not only is every motion from God as from the First Mover, but all
formal perfection is from Him as from the First Act. And thus the act
of the intellect or of any created being whatsoever depends upon God
in two ways: first, inasmuch as it is from Him that it has the form
whereby it acts; secondly, inasmuch as it is moved by Him to act.

Now every form bestowed on created things by God has power for a
determined act, which it can bring about in proportion to its own
proper endowment; and beyond which it is powerless, except by a
superadded form, as water can only heat when heated by the fire. And
thus the human understanding has a form, viz. intelligible light,
which of itself is sufficient for knowing certain intelligible
things, viz. those we can come to know through the senses. Higher
intelligible things the human intellect cannot know, unless it be
perfected by a stronger light, viz. the light of faith or prophecy
which is called the "light of grace," inasmuch as it is added to
nature.

Hence we must say that for the knowledge of any truth whatsoever man
needs Divine help, that the intellect may be moved by God to its act.
But he does not need a new light added to his natural light, in order
to know the truth in all things, but only in some that surpass his
natural knowledge. And yet at times God miraculously instructs some
by His grace in things that can be known by natural reason, ev