Thomas Aquinas' Summa Theologica: Being, Grace, Ethics, and the Order of Eternity

I. Introduction: The Purpose and Structure of Summa Theologica

In the prologue to the Summa Theologica, Thomas Aquinas clearly states the purpose of his work:

“To provide a theological textbook for beginners.” (Summa Theologiae, Prologus)

This vast work is composed of three major parts:

• Part I (Prima Pars) deals with God, creation, and human nature.

• Part II (Prima et Secunda Secundae) explores moral life and the virtues.

• Part III (Tertia Pars) addresses Christology, the sacraments, and the fulfillment of salvation.

Aquinas understands theology as “an intellectual movement that proceeds from God and returns to God,” encompassing ontology, epistemology, ethics, and soteriology.

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II. Rational Demonstration of God's Existence: Part I, Question 2

Aquinas opens Part I, Question 2 with the question:
“Is the existence of God self-evident?” He rejects Anselm’s ontological proof, favoring an inductive approach from the world of sense experience.

“The existence of God is not self-evident, for it requires a composite concept.” (ST, I, q.2, a.1)

Aquinas offers the famous Five Ways (quinque viae) to demonstrate God's existence:

1. From Motion: All motion requires an external mover.

2. From Efficient Cause: Nothing can be the cause of its own existence.

3. From Contingency: All beings are contingent; therefore, a necessary being must exist.

4. From Degrees of Perfection: Imperfect beings point to the existence of the most perfect being.

5. From Teleology: The order in nature implies a guiding intelligence.

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III. The Distinction of Essence and Existence: God as Pure Being

Aquinas affirms that God’s essence is identical to His existence (ipsum esse subsistens).

“God exists by His own essence. He alone is being itself.” (ST, I, q.3, a.4)

This metaphysical doctrine synthesizes Aristotle’s concepts of form and matter with Augustine’s eternal truth. While creatures receive being, God is being itself—simple, immutable, and perfect.

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IV. The Ultimate End and Human Happiness (Beatitudo): Part I-II, Question 1

Happiness (beatitudo) is the ultimate end of human life. Aquinas asserts:

“Man necessarily desires happiness.” (ST, I-II, q.1, a.1)

However, happiness cannot be found in wealth, pleasure, honor, or knowledge. True happiness lies in the beatific vision—the direct contemplation of God by the intellect.

“Perfect happiness consists in the vision of God.” (ST, I-II, q.3, a.8)

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V. Natural Law and Moral Practice: Part I-II, Questions 90–97

Aquinas grounds moral order in natural law (lex naturalis). Human reason reflects the eternal law (lex aeterna) of God, and the principle “do good and avoid evil” is inherent in all people.

“The natural law is nothing other than the rational creature's participation in the eternal law.” (ST, I-II, q.91, a.2)

From natural law arise human law (lex humana) and divine law (lex divina). Civil laws are valid only when they conform to moral order.

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VI. Caritas and the Theological Virtues: Part II-II, Questions 23–27

The highest virtue in Aquinas’s moral framework is charity (caritas)—the supernatural love of God and neighbor.

“Charity is the supernatural love of God and neighbor.” (ST, II-II, q.23, a.1)

It is not a mere emotion, but a rational and ordered act of the will. Caritas shapes all other virtues and is essential for salvation.

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VII. Christology and the Logic of Redemption: Part III, Questions 1–59

In Part III, Aquinas examines the mystery of the Incarnation and Redemption. Concerning Christ’s nature, he writes:

“The Word became man because the human nature was united with the divine person.” (ST, III, q.2, a.2)

Redemption is not a mere transaction but a manifestation of divine justice and mercy. The Cross is both satisfaction for sin and the gateway of grace enabling human freedom.

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VIII. Conclusion: Theological Intelligence as Integration

Summa Theologica is not simply a theological manual; it is an intellectual project of integrating reason and faith, nature and grace, philosophy and revelation, man and God.

“Grace does not destroy nature but perfects it.” (Gratia non tollit naturam sed perficit)

Aquinas's work represents the pinnacle of medieval Christian thought and continues to influence theology, philosophy, ethics, and political theory today.