Objection 3. Further, as shown above (Question 3, Article 4),
God's essence is existence. But existence seems most imperfect,
since it is most universal and receptive of all modification.
Therefore God is imperfect. I answer that, As the Philosopher
relates (Metaph. xii), some ancient philosophers, namely, the
Pythagoreans and Leucippus, did not predicate "best" and "most
perfect" of the first principle. The reason was that the ancient
philosophers considered only a material principle; and a
material principle is most imperfect. For since matter as such
is merely potential, the first material principle must be simply
potential, and thus most imperfect. Now God is the first
principle, not material, but in the order of efficient cause,
which must be most perfect. For just as matter, as such, is
merely potential, an agent, as such, is in the state of
actuality. Hence, the first active principle must needs be most
actual, and therefore most perfect; for a thing is perfect in
proportion to its state of actuality,