m this act of understanding, He understands that He
understands Himself; and so on to infinity. Therefore the act of God's
intellect is not His substance.

_On the contrary,_ Augustine says (De Trin. vii), "In God to be is
the same as to be wise." But to be wise is the same thing as to
understand. Therefore in God to be is the same thing as to
understand. But God's existence is His substance, as shown above
(Q. 3, A. 4). Therefore the act of God's intellect is His substance.

_I answer that,_ It must be said that the act of God's intellect is
His substance. For if His act of understanding were other than His
substance, then something else, as the Philosopher says (Metaph.
xii), would be the act and perfection of the divine substance, to
which the divine substance would be related, as potentiality is to
act, which is altogether impossible; because the act of understanding
is the perfection and act of the one understanding. Let us now
consider how this is. As was laid down above (A. 2), to understand is
not an act passing to anything extrinsic; for it remains in the
operator as his own act and perfection; as existence is the
perfection of the one existing: just as existence follows on the
form, so in like manner to understand follows on the intelligible
species. Now in God there is no form which is something other than
His existence, as shown above (Q. 3). Hence as His essence itself is
also His intelligible species, it necessarily follows that His act of
understanding must be His essence and His existence.

Thus it follows from all the foregoing that in God, intellect, and
the object understood, and the intelligible species, and His act of
understanding are entirely one and the same. Hence when God is said
to be understanding, no kind of multiplicity is attached to His
substance.

Reply Obj. 1: To understand is not an operation proceeding out of the
operator, but remaining in him.

Reply Obj. 2: When that act of understanding which is not subsistent
is understood, something not great is understood; as when we
understand our act of understanding; and so this cannot be likened to
the act of the divine understanding which is subsistent.

Thus appears the Reply to the Third Objection. For the act of divine
understanding subsists in itself, and belongs to its very self and is
not another's; hence it need not proceed to infinity.
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FIFTH ARTICLE [I, Q. 14, Art. 5]

Whether God Knows Things Oth