several ways. First, whatever a thing has besides its essence must be
caused either by the constituent principles of that essence (like a
property that necessarily accompanies the species--as the faculty of
laughing is proper to a man--and is caused by the constituent
principles of the species), or by some exterior agent--as heat is
caused in water by fire. Therefore, if the existence of a thing
differs from its essence, this existence must be caused either by some
exterior agent or by its essential principles. Now it is impossible
for a thing's existence to be caused by its essential constituent
principles, for nothing can be the sufficient cause of its own
existence, if its existence is caused. Therefore that thing, whose
existence differs from its essence, must have its existence caused by
another. But this cannot be true of God; because we call God the first
efficient cause. Therefore it is impossible that in God His existence
should differ from His essence. Secondly, existence is that which
makes every form or nature actual; for goodness and humanity are
spoken of as actual, only because they are spoken of as existing.
Therefore existence must be compared to essence, if the latter is a
distinct reality, as actuality to potentiality. Therefore, since in
God there is no potentiality, as shown above (A. 1), it follows
that in Him essence does not differ from existence. Therefore His
essence is His existence. Thirdly, because, just as that which has
fire, but is not itself fire, is on fire by participation; so that
which has existence but is not existence, is a being by participation.
But God is His own essence, as shown above (A. 3); if, therefore, He
is not His own existence He will be not essential, but participated
being. He will not therefore be the first being--which is absurd.
Therefore God is His own existence, and not merely His own essence.

Reply Obj. 1: A thing that has nothing added to it can be of