Point of Culmination
“This point is found in the idea of self-subsistent being. This idea unifies the five ways as a common keystone unifies five arches. Five attributes appear, one at the end of each way, in ascending order thus: first mover of the universe, corporeal and spiritual, first efficient cause, first necessary being, supreme being, supreme directing intelligence. Now these five attributes are to be found only in self-subsistent being, who alone can say: ‘I AM WHO AM.’ Let us look at each of the five.
The prime mover must be his own activity. But mode of activity follows mode of being. Hence the prime mover must be his own subsistent being.
The first cause, being uncaused, must have in itself the reason for its existence. But the reason why it cannot cause itself is that it must be before it can cause. Hence, not having received existence, it must be existence.
The first necessary being also implies existence as an essential attribute, that is, it cannot be conceived as merely having existence, but must be of itself existence.
The supreme being, being absolutely simple and perfect, cannot have a mere participated share of existence, but must be of itself existence.
Lastly, the supreme directing intelligence cannot be itself proportioned to an object other than itself; it must itself be the object actually and always known. Hence it must be able to say, not merely ‘I have truth and life,’ but rather ‘I am truth and life.’
Here, then, lies the culminating keystone point, the metaphysical terminus of the road that ascends from the sense world to God. This ascending road ends where begins the higher road, the road of the wisdom which, from on high, judges the world by its supreme cause.
Thus again, at the summit of the universe reappears the fundamental Thomistic truth. In God alone are essence and existence identified. In this supreme principle lies the real and essential distinction of God from the world. This distinction reveals God as unchangeable and the world as changeable (the first three proofs for His existence). It becomes more precise when it reveals God as absolutely simple and the world as multifariously composed (fourth and fifth proofs). It finds its definitive formula when it reveals God as “HE WHO IS,” whereas all other things are only receivers of existence, hence composed of receiver and received, of essence and existence. The creature is not its own existence, it has existence after receiving it. If the verb ‘is’ expresses identity of subject and predicate, the negation ‘is not’ denies this identification.
This truth is vaguely grasped by the common sense of natural reason, which, by a confused intuition, sees that the principle of identity is the supreme law of all reality, and hence the supreme law of thought. As A is identified with A, so is supreme reality identified with absolutely one and immutable Being, transcendentally and objectively distinct from the universe, which is, essentially diversified and mutable. This culminating point of natural reason, thus precisioned by philosophic reason, is at the same time revealed in this word of God to Moses: ‘I AM WHO AM.’
Now we understand the formulation given to the twenty-third of the twenty-four theses. It rungs thus: The divine essence, since it is identified with the actual exercise of existence itself, that is, since it is self-subsistent existence, is by the that identification proposed to us in its well-formed metaphysical constitution, and thereby gives us the reason for its infinite perfection. To say it briefly: God alone is self-subsistent existence, in God alone are essence and existence identified. This proposition, boundless in its range, reappears continually on the lips of St. Thomas. But it loses its deep meaning in those who, like Scotus and Suarez, refuse to admit in all creatures a real distinction between essence and existence.
To repeat. According to St. Thomas and his school God alone is His own existence, uncaused, unparticipated self-existence, whereas no creature is its own existence; the existence it has is participated, received, limited, by the essence, by the objective capacity which receives it. This truth is objective, a reality which antecedes all operation of the mind. Hence the composition of essence and existence is not a mere logical composition, but something really found in the very nature of created reality. Were it otherwise, were the creature not thus composed, then it would be act alone, pure act, no longer really and essentially distinct from God.
Self-existent understanding is given by some Thomists as the metaphysical essence of God, as the point where the five ways converge and culminate. While we prefer the term self-existent being, self-existent existence, the difference between the two positions is less great than it might at first seem to be. Those who see that culminating point in ipsum esse subsistens, begin by teaching that God is not body but pure spirit. From the spirituality follow the two positions in question: first, that God is the supreme Being, self-existent in absolute spirituality at the summit of all reality; second, that He is the supreme intelligence, the supreme truth, the supreme directive intelligence of the universe.
On this question, then, of God’s metaphysical essence according to our imperfect way of understanding, the two positions agree. They agree likewise when the question arises: What is it that formally constitutes the essence of God as He is in Himself, as He is known by the blessed in heaven who see Him without medium, face to face? The answer runs thus: Deity itself, not self-subsistent existence, not self-existent understanding. Self-subsisting existence indeed contains all divine attributes, but only implicitly, as deductions to be drawn therefrom in order, one by one. But Deity, God as He is in Himself, contains in transcendent simplicity all these divine attributes explicitly. The blessed in heaven, since they see God as He is, have no need of progressive deduction.”
– Lucas G. Westman
 Reality: A Synthesis of Thomistic Thought, Pg. 67-70