Can We Know God by Reason Alone?

by Lisa Quintana

When I ask students how can we know things, it’s assumed that knowledge is primarily gained through the Scientific Method.1 But when I say that’s not the only way to know certain things, I get blank stares. There is another way to know stuff — through reasoning skills. Sadly, those skills have been broadly underdeveloped. However, by using logic, we may be able to answer one of the most important questions of all time…

Nearly one-thousand years ago, a brilliant monk named Anselm believed one could convince themselves of the truth of God’s existence purely through the mind – using reason, logic and rationality. One of the arguments Anselm developed to show this is called the Ontological Argument. I know, that sounds like a mouthful, but ontology is a branch of metaphysics dealing with the ‘nature of being.’ Philosopher J.P. Moreland once described it as the “what-ness” of something, i.e., what kind of being is God?

Born in 11th Century, in his adolescence, Anselm decided that there was no better life than the monastic one. He sought to become a monk, but was refused by the Abbot of the local monastery. Leaving his birthplace as a young man, he headed north across the Alps to France, eventually arriving at Bec in Normandy, where he studied under the Benedictine monk and Archbishop of Canterbury. He became a monk at 27 years old. 

St. Anselm

What motivated Anselm to this monastic lifestyle at a relatively young age? He was interested in training minds in ways to foster both spiritual and intellectual development. His theological mentor was St. Augustine and like him, Anselm had a passion for solving intellectual problems.

“I do not try, Lord, to attain Your lofty heights, because my understanding is in no way equal to it. But I do desire to understand Your truth a little, that truth that my heart believes and loves. For I do not seek to understand that I may believe, but I believe in order to understand.”

– Anselm’s Proslogion

In 1070, Anselm began to write out of conflict in discourse from opposing views to compose answers to clarify his thoughts, recording his arguments. Conflicting ideas can be good, when done respectfully (unlike our modern ‘cancel culture’). Eventually, his teaching and thinking culminated in a set of treatises and dialogues. In 1077, he produced the Monologion, and in 1078 the Proslogion, or the Ontological Argument.

Anselm used deductive reasoning (logic) to argue the existence of God. Until the 11th C, there had been little study of logic in the Church since the first few centuries (CE). He developed the Scholastic Form where one would state a proposition and then explain why the proposition is true.

Anselm pondered Psalm 14:1, “The fool says in his heart, “There is no God.”’ This led him to try and understand if atheism is foolish. Unlike today, in Anselm’s day there were no overt atheists because atheism was considered blasphemy by the Church. The Church and State cooperated to enforce blasphemy laws. (That was not a good moment in church history.) Nonetheless, Anselm was interested in answering the question of why atheism is foolish.

Anselm created the first systematic treatise of Natural Theology: theology or knowledge of God based on observed facts and experience apart from divine revelation.2 He was curious about whether an airtight logical proof of God’s reality could be developed that would in no way depend on faith in divine revelation. He used the Socratic Dialogue Format as he attempted to use logic to strengthen faith.3

What Kind of Being is God

Every person, at one time or another, considers whether God exists. It’s in the contemplation, which is a universal thought, which Anselm begins his arguments. A key understanding to the Ontological argument is how one defines God. 

Anselm asks this basic question: “What is good?” He argues from the goodness of God —

1. The cause of this goodness is either one or many.

2. But it can’t be many, for then there would be no way to compare their goodness, for all things would be equally good. 

3. But some things are better than others.

4. Therefore, one Supreme Good (God) causes the goodness in all good things.

Every good thing given is from above, coming down from the Father of Lights,
with whom there is no variation or shifting shadow.”
— James 1:17 (NASB)

Necessary Entity

Anselm states that God’s existence is necessary4 because of the degrees of goodness in creation. He said that God must be perfectly good; the highest being; the root of all goodness. Anselm presupposes that people can, and do, distinguish between greater and lesser goods in life; it’s a universal objective standard of goodness.

Unpacking the famous “Ontological Argument”

The Proslogian is a single argument to prove that God really exists, that He is the ‘supreme good’, needing no other (self-existent), and is whom all things have need of for existence (contingent).

Anselm defines God as “The Greatest Conceivable Being.” Modern philosophers have adapted this term and renamed it “Maximally Great Being,” and I will use that modern term (MGB) in this revised version of his argument. This MGB encompasses these ‘immaterial’ realities5:

1. Necessary (in that it is the foundation of all created things)
2. Uncaused (self-existent)
3. Good
4. Wise
5. Just
6. Love, etc. (how we define God – omnipresent, omniscient, omnibenevolent)

In scholastic fashion, let’s jump into some of how this reasoning works…

Anselm’s Ontological Argument

Premise 1 – It is possible that God (defined as a “Maximally Great Being”6) exists.

Premise 2 – If it is possible that a maximally great being (MGB) exists, then a MGB exists in some possible worlds. 

Premise 3 – If a MGB exists in some possible worlds7, then a MGB exists in all possible worlds.

Premise 4 – If a MGB exists in all possible worlds, then a MGB exists in the actual world.

Premise 5 – If a MGB exists in the actual world, then a MGB (aka God) exists.

The Ontological argument is not typically understood the first time. It takes some reflection. It helps to know something about logic.

Logic derives from order. Without order, nothing makes sense (including existence). Logic uses propositions that are possible, impossible, necessary and contingent (dependent for existence, conditional). Logical possibilities can deal with the immaterial aspects of knowledge, i.e. abstracts like numbers. Is there a number 3 walking around out there? No. It is an abstract ‘immaterial reality’ that describes something. Logically coherent is what a MGB would be because a MGB would not do illogical absurdities, (i.e. like a square circle, the shape of purple, or a triangle with 4 sides).

“… it is impossible for God to lie,” Hebrews 6:18

“God cannot be tempted with evil,” James 1:13

If God was not necessary but contingent (dependent on something else for existence), then that implies God has a Creator, which is not logical if God is the MGB (or the greatest conceivable being). Who made God is asking who is greater than the MGB — incoherent. If God exists, He is a necessary being: not contingent on anything or anyone.

Conceptualizing God

If the concept of God does not contain any logical absurdities, then it is possible that God exists. Being ‘necessary’ is true in all possible realities. This idea aligns with the Prime Mover concept from ancient Greek Philosopher, Aristotle (that which moves without being moved). This is what some call the “uncaused Cause.”

But is God logically impossible?
J.L. Mackie (c. 1950s atheist philosopher) argued yes, God is logically impossible based on the problem of evil. Mackie argued that it’s a logical contradiction of an all-good being with the existence of evil

In the 1970s, Christian Philosopher Alvin Plantinga, currently one of the world’s most influential philosophers, refutes Mackie’s problem of evil argument with the Free Will Defense. In other words, it is possible that an all-good God can exist with evil since people have free will, and history has proven we can do some pretty evil things with it. God created humans in His image and that includes choices (a.k.a. free will). So God did not create evil; evil is a corruption of goodness. Thus, in keeping with Anselm’s idea that as long as it is logically possible, then it follows God exists.

Some call Anselm a “thinker who is always relevant.” C. S. Lewis was also influenced by Anselm’s concept of the God being the root of ALL goodness, emulated in Mere Christianity:

1. Some beings are more nearly perfect than are others.

2. But things cannot be more or less perfect unless there is a wholly perfect to which they can be compared.

3. Therefore, there must be a Most Perfect Being (God).

Anselm brought the use of reason (logic) back into Church theology. His Ontological Argument is his most famous argument, and it is still being used today (in various formats/revisions). 

So, can we know God by reason alone? What do you think?

Psalm 19:1-2, “The heavens declare the glory of God, and the sky above proclaims his handiwork. Day to day pours out speech, and night to night reveals knowledge.”


  1.  Knowledge based on demonstrable and reproducible data.
  2.  Roger Olson, The Story of Christian Theology, (Illinois: IVP Academic, 1999), 318.
  3.  A small group (5-15 people), guided by a facilitator, finds a precise answer to some universal questions.
  4.  A necessary being is one that is not contingent upon anything else; God is self-existent, the uncaused-cause of it all.
  5.  Immaterial realities need to be known using philosophical logic; the scientific method can only measure material realities. 
  6.  A MGB has to be a necessary entity that exists because a contingent being cannot be maximally great.
  7.  “Possible worlds” is a philosophical term used to express a way reality could be (various realities). Our actual world is one of these “coulds”.


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