Does God Exist? How Do I Know? The Five Ways of Aquinas – Fr. Irenaeus Dunlevy, O.P.

Jan 26, 2026

Fr. Irenaeus Dunlevy, O.P., a Dominican priest and campus outreach coordinator with a background in architecture and campus ministry, walks through Aquinas' Five Ways. He outlines proofs from motion, causation, necessity, gradation, and teleology. Short, clear explanations highlight what those arguments aim to show and where reason stops and faith begins.

59:45

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ANECDOTE

Transformative Encounters With The Five Ways

Fr. Irenaeus Dunlevy shares that encountering Aquinas' Five Ways converted an atheist friend who later became a Dominican nun.
He also recounts how the Five Ways launched Bishop Barron's intellectual vocation in high school.

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INSIGHT

Reason From Effects, Not From Self‑Evidence

Aquinas uses a posteriori arguments: we infer God from observable effects, not from self-evident knowledge.
Each of the Five Ways starts from different observable features and defines 'God' differently.

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ADVICE

Respect Intelligence In God Debates

When discussing God's existence, avoid claiming it's self‑evident to others.
Respect opponents' intelligence and use reasoned, a posteriori arguments instead.

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Purpose: Proving God's Existence

00:25 • 3min

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Clarifying the Scope of the Five Ways

03:52 • 6min

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Reason, Faith, and Knowing God

09:23 • 3min

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Overview: The Five Ways Summarized

12:02 • 1min

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First Way: Motion and Unmoved Mover

13:30 • 8min

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Second Way: Efficient Causes and First Cause

21:49 • 11min

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Third Way: Possibility, Necessity, and Necessary Being

32:24 • 8min

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Fourth Way: Gradation to a Maximum Being

40:38 • 10min

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Fifth Way: Teleology and Intelligent Governance

50:46 • 7min

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Limits of the Five Ways and Further Inquiry

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Fr. Irenaeus Dunlevy introduces Aquinas’ Five Ways, showing how arguments from motion, causality, contingency, gradation, and teleology lead from everyday experience to the rational conclusion that God exists as first mover, first cause, necessary being, supreme perfection, and intelligent governor.

This lecture was given on May 8th, 2025, at North Dakota State University.

For more information on upcoming events, visit us at thomisticinstitute.org/upcoming-events.

About the Speakers:

Fr. Irenaeus Dunlevy, O.P. is a Coordinator for Campus Outreach at the Thomistic Institute in Washington, DC. He has served as a parochial vicar at St. Pius V Church in Providence, RI, as well as an adjunct professor and assistant chaplain at Providence College. He originates from Columbus, OH, studied architecture in Virginia and Switzerland, and practiced in the DC area before entering the Order of Preachers in 2013. He was ordained a priest in 2020 at the Dominican House of Studies during the quarantine. In his work with the Thomistic Institute, he has given talks on the virtue of penance, loving God with the mind, and the intersection of theology and architecture. He often travels the country visiting Thomistic Institute Campus Chapters, leading seminars that help students grasp Thomistic concepts. Additionally, he coordinates the TI's intellectual retreat programming, which affords students time to pray and integrate into their lives Thomistic theology and philosophy.

Keywords: Act and Potency, Argument from Motion, Contingency and Necessary Being, Degrees of Perfection and Maximum Good, Five Ways to Prove God’s Existence, First Cause and Efficient Causality, Teleology and Intelligent Governor, Thomistic Proofs for God, Unmoved Mover

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