The Five Ways of Thomas Aquinas: Proving God’s Existence
Theology: Philosophical and Revealed
Since its inception, theology has encompassed philosophical reflection and rational inquiry into the existence of a First Cause. This First Cause is often identified with a being that shapes the world. Revealed theology, on the other hand, focuses on philosophical reflection on the contents of Revelation. It emerged in early Christian thought and developed during the Middle Ages, drawing heavily on Augustinian and Platonic philosophy.
The recovery of Aristotle’s works by Averroes led to the philosophical treatment of themes central to Augustinian theology, such as the existence and properties of God, creation, the immortality of the soul, and more.
Thomas Aquinas distinguished between two types of theology: sacred (or supernatural) theology and philosophical (or natural) theology. Both rely on rational discourse.
Rational Theology
Rational or philosophical theology investigates divine realities as the cause of the world. This science, practiced by philosophers, is rooted in metaphysics. By investigating the root causes of existence, we encounter the foundation of all that is – God. Thus, metaphysics delves into discussions about God. Theology is possible because humans can investigate reality scientifically and rigorously, starting from sensory knowledge and seeking rational causes and principles until reaching the First Principle, which is God.
Natural and philosophical knowledge of God is based on our understanding of the world. We can reach God as the First Cause, the ordering intelligence, and the ultimate end. By examining the effects (the world and its beings), we can trace back to their cause (God) and learn something about it.
Revealed Theology
Revealed, sacred, or supernatural theology also aims to study God, but its source is not natural knowledge but the book of Revelation (grace). It explores the intimacy of God revealed to humanity, including concepts like the Trinity and the Incarnation. Using logic and reason, supernatural theology reflects on the book of Revelation, making its contents more understandable and allowing faith to transcend our natural knowledge.
Reconciling Philosophical and Revealed Theology
Despite their distinctions, Thomas Aquinas recognized that conflicts could arise between philosophical and revealed theology. To address this, he differentiated between the preambles of faith and the articles of faith.
Preambles of faith are truths that can be known philosophically but are also revealed (e.g., God’s existence, creation). They are common themes in both philosophical and revealed theology.
Articles of faith (or mysteries of faith) are revealed truths accepted by faith and are the sole domain of revealed theology (e.g., the Trinity, Incarnation, Resurrection).
Thomas Aquinas emphasized the cooperation and agreement between the two theologies.
Collaboration: Rational theology leads to faith through its preambles, which act as a preparatory path. While it’s not necessary for individuals to engage in philosophical investigations to have faith, faith can serve as a shortcut to belief. Revelation is inherently rational, but due to human limitations, we can only demonstrate the preambles of faith.
Coincidence: There is only one truth, revealed by God through faith (an infallible supernatural gift) and partially accessible through reason (a natural human attribute). However, if philosophers or scientists reach conclusions that contradict faith, faith should act as a corrective standard. Thomas Aquinas criticized Aristotle for asserting ideas like the eternity of the world and the denial of the soul’s immortality.
The Five Ways and Argumentative Structure
The “Five Ways” are arguments presented by Thomas Aquinas in the Summa Theologica to prove God’s existence. The Summa Theologica (ST) is a monumental work of philosophical and sacred theology, employing the scholastic method of quaestiones (disputations). The Five Ways are part of the purely philosophical expositions within the ST.
The ST has a structured format, divided into three parts, each developed through questions and subdivided into articles. Each article follows a specific structure:
- Presentation of the problem: Often framed as an alternative or dilemma (e.g., Does God exist or not?).
- Procedural disputes: Examination of reasons for and against the proposition. Reasons against Thomas’s thesis are presented as “objections,” while those in favor are introduced with “on the contrary.”
- Body of the article (solution): Presents Thomas’s argument and philosophical conclusion, often based on demonstration.
- Response to objections: Addresses the initial objections based on the thesis outlined in the solution.
The text being discussed is Article 3 of Question 2 of the First Part of the ST (ST, I, q2, a3). Question 2 explores whether God exists, with Article 1 examining if God’s existence is self-evident and Article 2 discussing if it’s demonstrable.
Is God’s Existence Self-Evident?
According to Thomas Aquinas, the proposition “God exists” is not self-evident to us. For a proposition to be self-evident, the predicate must be logically included in the subject’s notion, or the denial of the proposition must be contradictory. Neither condition is met in this case. We cannot know if “exists” is an essential part of “God” because it would require complete knowledge of the divine essence, which is beyond human understanding. Furthermore, “God does not exist” is as logically valid as “God exists,” so God’s existence cannot be considered self-evident.
Is God’s Existence Demonstrable?
In Article 2, Thomas Aquinas considers two types of demonstration: a priori (propter quid) and a posteriori (quia).
A priori demonstration derives knowledge of something’s properties from its essence. For example, from the definition of a circle, we can conclude that all its radii are equal. In the case of God’s existence, an a priori proof would establish existence based on the essence or notion of God. Thomas rejects this approach because, as mentioned, knowledge of God’s essence is beyond human comprehension. This type of proof, used by St. Anselm and Descartes, is known as the “ontological argument.”
A posteriori demonstration starts from observed facts or effects and traces them back to the necessary existence of a cause, even without full knowledge of the cause’s essence. This is the only suitable method for proving God’s existence, as it relies on worldly events (effects) and infers the necessary existence of a First Cause, which aligns with the revealed notion of God.
The Five Ways
Thomas Aquinas developed five a posteriori proofs – the “Five Ways” – all sharing a common structure:
- Starting point: A universal effect evident in the beings that are the subject of human knowledge, based on sensory experience. The starting points are: motion (1st Way), efficient causality (2nd Way), contingency (3rd Way), gradation of perfection (4th Way), and teleology (5th Way).
- Principle of efficient causality: Applied to the starting point. Each Way formulates the principle differently, depending on the type of effect considered. Examples include: everything that moves is moved by another (1st Way), nothing is the cause of itself (2nd Way), every contingent being has a need in another (3rd Way).
- Corollary: It’s impossible to have an infinite regress in the series of subordinate causes. Without a First Cause, the entire series wouldn’t exist, implying the absence of the observed effects, which is absurd.
- Conclusion: Establishes the existence of a First Cause, specific to each Way’s starting point and principle of causality. Examples include: the First Unmoved Mover (1st Way), the Uncaused Efficient Cause (2nd Way), the Absolutely Necessary Being (3rd Way).
- Identification: Identifies the First Cause with the meaning of “God” as understood through faith.
The rational demonstration in each Way encompasses steps 1 to 4. The conclusion cannot be “God exists” in a purely rational sense because that would require the premises to be prior, better known, and causally related to the conclusion, implying a complete understanding of God’s essence, which is impossible.
Reason alone can prove the existence of a First Cause within the physical or natural realm. However, since the rational conclusion (“there is a First Cause”) is consistent with the revealed understanding of God, the Five Ways confirm what faith already teaches. The 5th step is a leap from reason to faith – it is reasonable to believe in God’s existence.