The Unmoved Mover – Thomas Aquinas on God's Existence, Divine Simplicity & the Trinity
The Unmoved Mover: Establishing the Existence of God
All things in motion must be put in motion, and it is impossible for there to be an infinite regress of movers, since each mover would need to be moved by another, resulting in an infinite chain without any actual movement. Therefore, there must be a first, unmoved mover, which we understand to be God. This first mover is not made of matter, for matter itself is subject to change and requires a beginning. Since matter is movable, the immovable mover must be immaterial.
Since God is the first cause, the source of all motion and change, He must be outside of the created order, not made of matter, and therefore eternal and immutable. His immutability ensures that He is not subject to the limitations that govern created things. His knowledge of all things must be perfect, for He is the source of all things. God's intellect is identical with His essence, and He understands Himself perfectly as the cause of all creation, meaning that His knowledge is not separate from His being.
God as the Ultimate Good and Source of All Perfection
As the first cause of all things, God is the ultimate good, and all things desire to return to Him, their origin, because all things are drawn toward their perfection. God, therefore, must be the very essence of goodness, since He is the cause of all that is good. The perfection of all things flows from His goodness, and as such, He is the standard of what it means to be good.
Moreover, since God is the first mover and is not in need of any external cause, His wisdom must also be perfectly unified with His being. His knowledge, which created all things, is not an external principle, but the very essence of who He is. This wisdom, as God's perfect knowledge, must be eternal and unchanging, and so it is one with God Himself.
Divine Simplicity and the Relationship Between the Father, Son, and Holy Spirit
For God to create and move all things, He must, in some sense, be present in them, but since He is immaterial, He cannot be present in the same way that physical beings are. Instead, His wisdom and His will are present in the things He has created, sustaining them and moving them toward their fulfillment.
The relationship between the Father (who is the source), the Son (who is the Word or wisdom of God), and the Holy Spirit (who is the love or will binding them together) is a perfect expression of God's nature. In this relationship, the love between the Father and the Son is so perfect that it is the very Spirit, and this love is the ultimate fulfillment of all things.
Since God is simple, His wisdom and love are not separate from His essence, but are perfectly united in Him. The internal relationship between the Father, Son, and Holy Spirit reflects the perfection of God’s goodness, knowledge, and love. This divine dynamic is a reflection of the perfect unity within God, and it is through this unity that He causes all things to be.
The Eternal Procession of the Son as the Word of God
Summa Theologiae I, Question 27, Article 1
How can we know this for certain though? Well In John 8:42 When Jesus states, “From God I proceeded,” the procession He refers to must be understood in a way that aligns with God’s immutability, simplicity, and perfection.
In God, there is no external or physical movement, nor any change or division within His essence. Instead, as Aquinas explains, procession in God is entirely internal and intellectual, reflecting the perfect and eternal nature of the divine essence.
In human terms, we see a procession within the intellect when a person conceives a thought or idea. This "word" of the intellect, while proceeding from the mind, remains within the thinker, united with their essence.
Similarly, in God, the Word proceeds—not as something external or separate—but as a perfect expression of God’s intellect. This Word is the Son, the second person of the Trinity, begotten eternally by the Father.
The procession of the Word in God demonstrates His perfect self-understanding. As God contemplates Himself, the Word is generated as a perfect image of His essence.
This procession does not imply change or division, as it might in created beings, but rather reveals the fullness of God’s intellectual life. The Word, being perfectly one with the Father, shares the same divine essence, yet is distinct as the Son.
The Procession of the Holy Spirit as Divine Love
Summa Theologiae I, Question 27, Article 3
Furthermore, Aquinas posits another procession in God: the procession of love. Love, as an operation of the will, reflects the mutual giving and receiving between the Father and the Son.
This love is not merely an attribute of God but is so perfect and substantial that it is the Holy Spirit, the third person of the Trinity. The Holy Spirit proceeds from the Father and the Son, not as something external but as the bond of love that unites them.
This procession, like that of the Word, remains internal to God and reflects His eternal and unchanging nature.
These two processions—of the Word and of love—demonstrate the perfect unity and relationality within the Godhead. The Father, Son, and Holy Spirit are distinct persons, yet they are perfectly one in essence.
This is why the Trinity does not violate God’s simplicity; the processions are internal acts that fully express His essence without introducing division or multiplicity. The unity of the Trinity reflects the infinite perfection of God’s knowledge and love.
This understanding is supported by Scripture, as in John 1:1, where it is written, “In the beginning was the Word, and the Word was with God, and the Word was God.”
Here, the Word (the Son) is both distinct from and one with the Father. Similarly, in John 15:26, Jesus speaks of the Holy Spirit as proceeding from the Father, emphasizing the relational dynamic within the Trinity while affirming the unity of their divine essence.
The Trinity as Three Distinct Persons in One Divine Essence
Summa Theologiae I, Question 29, Article 1
Thus, the doctrine of the Trinity presents a profound explanation of how God can be both one and relational. The Father, Son, and Holy Spirit are not three gods but three distinct persons united in one divine essence.
This relational unity does not compromise God’s simplicity, immutability, or perfection but rather demonstrates the fullness of His being.
The Father, as the unoriginated source, eternally generates the Son, the perfect Word, through an intellectual procession. The Father and Son together breathe forth the Holy Spirit, the love proceeding from their mutual self-giving.
These processions are internal and eternal, meaning they do not involve time, change, or external action. Instead, they reflect God's infinite self-knowledge and love, perfectly united in His single essence.
Creation, Human Nature, and the Image of God
This framework also explains why all things in creation are drawn toward God. Since God is the ultimate good and the source of all being, everything naturally desires to return to its origin and fulfillment in Him.
Creation reflects, in a finite way, the relational dynamic of the Trinity, as all things exist in relation to the Creator. The Father creates through the Son (the Word) and sustains all things in the Holy Spirit (love), continually moving creation toward its ultimate perfection in union with Him.
As creatures made in God’s image, humans have an intellect and will, enabling them to know and love. Through our intellect, we participate in the eternal procession of the Word, and through our will, we share in the procession of love.
This capacity to know and love God gives humanity a unique dignity and a special role in manifesting His glory. It is in this relational nature that the mystery of the Trinity is revealed most clearly in creation.
The Trinity and the Defense of Christian Monotheism
In light of this, the Trinity provides a rational framework not only for understanding God but also for engaging in dialogue with others who believe in one God, such as Muslims.
While the doctrine of the Trinity may seem paradoxical, it does not violate the principle of monotheism. Instead, it deepens our understanding of how the one God exists as a communion of persons, perfectly united in essence.
By showing that God’s essence is relational and self-giving, the Trinity offers a unique perspective on how unity and diversity can coexist harmoniously in the divine nature.
Why the Trinity Cannot Be Proven Through Natural Reason Alone
Summa Theologiae I, Question 32, Article 1
It is impossible to arrive at knowledge of the Trinity of divine Persons by natural reason. This is because is as humans can not truly understand something like there being 3 persons in one being. This is where God reveals the truth to us via Devine wisdom and through this we must have faith in the things believed but unseen.
Aquinas argues that God has revealed the Trinity to humanity not because it is necessary for understanding His existence, but because it reflects His inner life and perfect nature.
Through faith, aided by revelation, we come to know that God exists as three distinct persons—Father, Son, and Holy Spirit—yet remains one in essence. This truth surpasses human understanding and cannot be proven or deduced through reason alone.
Even though reason cannot fully comprehend the Trinity, Aquinas asserts that it does not contradict reason.
The doctrine of the Trinity aligns with the principles of divine simplicity and unity, as the distinctions of persons do not divide God's essence. Rather, the Father, Son, and Holy Spirit are relational distinctions within the one divine being.
The Holy Spirit's Procession from the Father and the Son
Summa Theologiae I, Question 36, Article 2
He explains that the Holy Spirit proceeds from the Father and the Son as the bond of love between them. This procession is distinct from the generation of the Son, which is intellectual, as the Holy Spirit proceeds through the will, specifically as love.
Aquinas emphasizes that this doctrine maintains the unity and simplicity of God. The Holy Spirit is not a separate essence but shares the same divine nature as the Father and the Son.
The relationship among the three persons reflects perfect unity, as the Father loves the Son, the Son loves the Father, and this love itself is the Holy Spirit.
This procession as love underscores the relational dynamic within the Trinity while preserving the oneness of God.
Divine Processions, Unity, and the Relationship Between Reason and Faith
Summa Theologiae I, Question 39, Article 7
We can further explore how the divine processions and the nature of God, as revealed through Scripture, align with reason and faith.
While we cannot fully comprehend the mystery of the Trinity with natural reason, this doctrine doesn't conflict with reason. Rather, it reflects the perfect internal life of God. The distinct persons—the Father, Son, and Holy Spirit—are relational within the one essence, each contributing to the unity of God.
We see this in the procession of the Son, the Word, which emanates from the Father through intellectual understanding. This procession is eternal, not involving any physical change or division within God, showing how the divine essence remains undivided while allowing for distinct persons to exist within the unity of the Godhead.
Similarly, the Holy Spirit's procession demonstrates the love that flows between the Father and the Son, perfectly uniting them in a bond of love.
This love is not an addition to God but is a reflection of His essence—God is love. The Holy Spirit, proceeding from both the Father and the Son, is not a separate being but is the mutual love that binds the Father and the Son together.
This eternal, internal procession reveals the depth of the relational nature within the Godhead.
These processions do not imply change, division, or imperfection, as God’s simplicity and immutability remain intact. This understanding allows us to see how God’s essence can be both unified and relational, a mystery revealed through divine wisdom and accessible to us through faith.
In light of this, the Trinity also provides a rational framework for understanding the coexistence of unity and diversity.
The divine unity in God is not undermined by the three distinct persons but is rather enhanced by the eternal relationships among them. This helps us understand how God can be one and yet relational, and it offers a profound way to engage with those who believe in one God, such as Muslims, demonstrating that the doctrine of the Trinity is not a contradiction to monotheism but a deepening of our understanding of the nature of the one true God.
Ultimately, this divine mystery is something revealed through Scripture, and though our reason cannot fully grasp it, faith allows us to believe in its truth.
The inner life of God, as revealed in the doctrine of the Trinity, shows that God is both relational and simple, a unity in diversity, offering a profound understanding of His perfection and love.
Divine Simplicity and the Unified Operations of the Trinity
Summa Theologiae I, Question 43, Article 2
Aquinas restates that God is simple—meaning that His essence is not composed of parts or divided into separate elements. His attributes, such as goodness, wisdom, and power, are not distinct from His essence but are identical with it.
This simplicity applies even to the relationships within the Trinity. The Father, Son, and Holy Spirit are distinct persons, but they share the same divine essence in such a way that their distinction does not imply division or multiplicity in God.
The relationships within the Trinity—be it the intellectual procession of the Son from the Father or the willful procession of the Holy Spirit from the Father and the Son—are eternal and do not introduce any change or division in God's unity.
Aquinas then delves into the idea of divine operations and the way in which the three persons of the Trinity work together in perfect harmony.
Since the three persons are one in essence, their operations are also one. For example, when God acts in creation, it is not the Father alone who creates, nor the Son or the Holy Spirit alone.
Rather, all three persons are involved, with the Father being the source, the Son being the Word or Wisdom through which creation is brought into being, and the Holy Spirit as the bond of love that sustains all things in existence.
This cooperative action is not contradictory to the oneness of God but is rather a reflection of the perfect unity and harmony within the Godhead.
The Human Soul as an Analogy for the Trinity
Aquinas uses the analogy of a human soul to explain this unity and cooperation, although he stresses that human analogies are imperfect and cannot fully capture the depth of the divine reality.
Just as the soul is a single substance but can have different powers (intellect, will, etc.), so too the Trinity is a single divine essence with three distinct persons, each with a role in the divine work of creation and redemption.
Moreover, Aquinas addresses the concern that the existence of three persons within the one God might imply some kind of division or imperfection.
He refutes this by emphasizing that God’s simplicity means there is no potentiality or composition in God.
The distinction of persons within the Godhead does not reflect any change or division in the divine nature; rather, it reflects the fullness of God’s eternal life and relationality.
In this way, the doctrine of the Trinity actually affirms God's perfection, for it shows that God’s nature is not static or solitary but is eternally relational and self-giving.
The Fullness of the Divine Essence in Each Person of the Trinity
Aquinas also touches on the relationship between God’s essence and the divine persons.
He argues that the essence of God is fully present in each of the three persons. The Father is not more or less God than the Son, and the Holy Spirit is not a lesser form of the divine essence.
The persons are fully divine in their own right, and yet, since they share the same divine essence, they are perfectly united in a single, undivided God.
This understanding reinforces the idea of divine simplicity and avoids any form of polytheism or division in the divine being.
In conclusion, Aquinas reaffirms that the Trinity, though a mystery beyond full human comprehension, is a coherent and consistent doctrine that aligns with the principles of divine simplicity, immutability, and unity.
The relationships within the Godhead—intellectual (the Father and the Son) and willful (the Father, Son, and Holy Spirit)—do not divide God’s essence but rather reflect the fullness of His eternal nature.
This relational unity is not a deficiency or imperfection but a perfect expression of the divine life, which is both one and relational, simple and profound, beyond the grasp of human reason but accessible through faith and revelation.
Thus, the doctrine of the Trinity provides a deeper understanding of God’s nature, His eternal self-giving love, and His perfect knowledge and wisdom, offering a foundation for both understanding and worshiping the one true God who exists in three persons.
Thomas Aquinas on the First Cause, Pure Actuality, and Divine Perfection
Thomas Aquinas presents a robust philosophical and theological argument for the doctrine of the Trinity, asserting that God is both one essence and three distinct persons.
At the core of his argument is the idea that there must be an initial, unmoved mover — a first cause that does not itself depend on anything else for its existence.
This unmoved mover is, by definition, simple and unchanging. God's immutability is crucial because it means that He is pure actuality, without potentiality or change.
Since change would imply a limitation, a being that changes cannot be the necessary first cause. The unmoved mover must therefore be perfect, eternal, and without division, reflecting the foundational understanding that God’s essence is simple.
Aquinas argues that in God, essence and existence are identical. Unlike creatures whose essence (what they are) and existence (that they are) are distinct, in God, His very being is identical with His essence.
This insight leads Aquinas to assert that God’s being is utterly perfect and complete. If God's essence were to differ from His existence, He would not be truly self-sufficient.
In other words, God's essence as being fully realized and necessary means He is pure act, lacking nothing. This self-sufficiency of God sets the stage for understanding the unity and the perfection that underpins the Christian doctrine of the Trinity.
Person and Substance: How God Can Be One and Three
The Trinity itself, according to Aquinas, is not a division of God's essence but a revelation of how God exists in three persons.
Each of the three persons — the Father, the Son, and the Holy Spirit — shares the same divine essence, but their distinctions lie in their internal relations, specifically in the processions.
The Son proceeds from the Father as the Word, the Father eternally knows Himself in the Son. The Holy Spirit proceeds as the love between the Father and the Son, an act of eternal love that binds them.
These processions are internal to God, meaning they do not imply change or division in His essence, but instead reflect the perfect and eternal relations that define the persons of the Trinity.
Aquinas' doctrine of divine simplicity also plays a crucial role in his understanding of the Trinity.
For Aquinas, divine simplicity means that God is not composed of parts or multiple aspects that could suggest a division within His essence. God’s essence is indivisible.
The persons of the Trinity, therefore, do not represent divisions in God’s essence but are rather distinctions of relationship.
The Father, Son, and Holy Spirit are not parts of God; rather, they are distinct persons who share one essence.
Aquinas thus argues that the doctrine of the Trinity is not a contradiction of God’s unity but rather a reflection of how divine unity can exist in a relational, personal way.
Aquinas uses the terms “substance” and “person” to further clarify this relationship.
While "substance" refers to the essence of God (that which He is), “person” refers to the distinct modes of being that exist within the one divine essence.
The Father is God as begetter, the Son as begotten, and the Holy Spirit as the bond of love between them.
These personal distinctions do not imply a division in God’s substance but are a way of articulating the relational dynamics within the divine life.
The Father’s begetting of the Son is an eternal act, not a temporal one, and the procession of the Holy Spirit is similarly eternal and not the result of any change or differentiation in God.
Aquinas also discusses the necessity of distinguishing between essence and existence when it comes to the Trinity.
The essence of God is simple and indivisible, but the three persons are distinguishable by their relational properties — not by differing essences or substances.
This distinction avoids the error of treating the persons as mere aspects of one God or as separate gods.
The Father, Son, and Holy Spirit are distinct, but they are not divided.
This crucial distinction supports the logical consistency of the doctrine of the Trinity, ensuring that it does not undermine God's unity while still allowing for the personal distinctions that Christians believe to be revealed in Scripture.
God's Perfect Knowledge and the Eternal Generation of the Word
In addition to divine simplicity, Aquinas emphasizes the perfection of God's knowledge.
Since God is simple and immutable, His knowledge is not the result of a process or change. God's knowledge is perfect and complete, encompassing all that exists, has existed, and will exist.
His knowledge of Himself and His creation is identical, meaning that He is perfectly self-aware.
In this light, the relationship between the Father and the Son — where the Son eternally knows the Father and vice versa — is an expression of the perfect, unchanging knowledge that defines God's nature.
The procession of the Son as the Word, then, is a manifestation of divine knowledge.
Divine Love and the Eternal Procession of the Holy Spirit
Finally, the work of the Holy Spirit can be understood as the love between the Father and the Son.
Aquinas underscores that love is not an additional attribute or division in God but is an essential act that, like the Word, is internal and eternal.
The love that proceeds from the Father and the Son is not a temporal event but is eternally present within the divine life.
This love is not external to God but constitutes part of His inner life, expressing the perfect relationship between the persons of the Trinity.
Conclusion: The Trinity as a Rationally Coherent Mystery of Faith
In conclusion, Thomas Aquinas’ argument for the Trinity shows that belief in one God in three persons is logically consistent with the unity and simplicity of God’s essence.
By differentiating between person and substance, Aquinas provides a coherent framework for understanding how God can be both one and three.
His emphasis on divine simplicity, immutability, and the internal processions of the persons of the Trinity allows for a rational understanding of the doctrine, helping to reconcile Christian faith with philosophical reasoning.
This framework also offers a model for engaging in apologetic dialogue with those, such as Muslims, who may find it challenging to understand how one God can exist in three persons.
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