Have you ever found yourself wondering if God exists? More precisely, wondering if there’s a way to know God exists without relying on blind faith? Could belief in God make sense on a strictly rational basis?

If so, you’re in good company — this has been an important question for many thinkers. One of the intellectual giants in the Church, St. Thomas Aquinas, articulated five ways we can know of God’s existence through reason and what we observe in the natural world. For more than 700 years, these five ways have served as a starting point for those struggling to believe in God and looking for a way to seek Him. 

Argument from Motion

When I teach the first of Aquinas’ five ways to high schoolers, I start by just throwing a crumpled up piece of paper at a favorite student. 

“What caused the paper to move?” I ask. 

“Your arm,” one student responds correctly. 

“Exactly — every movement is caused by something else,” I say. Newton came to the same conclusion: things don’t move by themselves — they only move when they are acted upon by an external force.  

Aquinas’ first way is an argument from motion: there must be a first mover. We live in a universe that is constantly moving — there must be something (or Someone) who initiated the first motion.

Argument from Causation

A student offered a different (and a little snarky) interpretation of what made the trash ball move:
“He probably did something to deserve it,” she said. She was already hinting at Aquinas’ second way, which is an argument from cause and effect. 

In addition to the idea that I physically acted upon the paper and it physically moved through the air (argument from motion), there’s also the idea that someone was responsible for the paper’s motion. I caused it to happen.

I make my case in a different way by walking to the light switch. I flip the switch and the lights go off. Flip again and they come on. The light didn’t change on its own — the electricity flowing to the bulb started and stopped. Why? Because I acted upon the light switch.

Everything in the universe appears to have been caused by something else. But what was the first cause? There must be Someone who started the whole chain of cause and effect that is behind everything that happens.

Argument from Necessary Being

“There was a time when I did not exist,” I say with an overly dramatic philosophical tone. “The same is true for you and for you,” I point to a few of my students, “and for this chair and for this desk and for this school and for Billie Eilish and for tacos and…” They get the point. Everything in the universe was once not in existence. 

“But nothing comes from nothing,” I say. “There must be something or Someone who existed before everything else so that everything else could come into existence.” 

Everything in the natural world depends on something else to be in existence. Billie Eilish wouldn’t be here without her parents, and her parents would be here without her grandparents — and on and on. But where did that chain of existence start? There must be one thing that does not depend on something else to exist — Aquinas argues that this points to the existence of God. 

Argument from Gradation

“Raise your hand if you are perfect and you can love everyone perfectly?” No responses. “Every one of us can love better tomorrow than we can today. There is always room for improvement.” 

We observe gradation in nature all the time: an apple doesn’t taste as sweet as honey. That’s a judgment we can make because we have a sense of sweetness. The same is true for transcendental realities like goodness and love. Aquinas argues that if there is always room to grow in goodness, then there must be an ultimate level of goodness by which all other good is measured: God is the ultimate level of goodness and love.

If there is no ultimate goodness, then we are free to define good and bad how we desire. This creates all sorts of logical problems, especially when one definition of goodness directly contradicts another. St. Thomas Aquinas reminds us that it is rational to believe in an ultimate level of goodness, and that ultimate level of goodness is not just an idea — it is a Person.

Argument from Design

Aquinas’ argument from design is that everything in nature seems to “know” what to do, even if it is not a living, thinking being. There is a “design” to everything. How do bees know how to build a hive? How do seeds know how to reach for sunlight when they are buried in the ground? How is it that the Earth has a perfectly suited atmosphere and climate to support life? If we look around at the world of nature, we can infer that there’s an intelligence behind its order and design — an intelligence that is reasonable to attribute to a Creator.

The more we understand the unique and special reality in which we live, and the more scientists study how improbable the conditions are for this creation we call home, the more we start to see some form of design or plan at work. But you don’t need to be a theoretical physicist to see this design; 700 years ago, St. Thomas Aquinas saw it in the simple observations of the plants, the animals, the minerals, and virtually everything around him. We can see that design, too.

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The Five Ways are tools that we can use to start thinking about God. I never actually intend my students to leave these lessons full of new-found faith with halos around their heads. But I do expect them to start using their heads when they talk about God. 

St. Thomas Aquinas gave us a way to critically analyze the world around us and to determine if there is any reason whatsoever to accept or to deny the existence of God. He is offering us a way to contemplate who God might be and how we can begin to know God, precisely using our observation and rational thinking. 

Once we begin that journey of seeking to know God, we discover He is more than an unmoved mover or a first cause or a cosmic watchmaker. Faith is not blind belief or an assent to a set of dogmas — it is a way of being in relationship with Him. And when we start looking at faith as a relationship and not simply an idea, we start to find ourselves not wondering if God exists, but, rather, experiencing wonder about God and His love for us.