Can Reason Know God?
Is believing in God just a leap in the dark?
Main teaching: De fide
The big question
Some people think believing in God is like choosing a favourite team: you just pick, and no reasons settle it. Others think science has made God unnecessary. The Catholic Church takes a position that surprises both: she says that God’s existence can be known, with certainty, by human reason. What does that mean, and why would the Church insist on it?
What the Church teaches
God, the beginning and end of all things, can be known with certainty by the natural light of human reason from the things he has made.
How sure is this? De fide. Defined by the First Vatican Council (Dei Filius ch. 2 and canon 2.1; DH 3004, 3026). The Oath against Modernism (1910) adds that God can be known and so also demonstrated, as a cause is known from its effects (DH 3538). See CCC 31–36.
In practice, people find this knowledge hard. Because of our weakness and the effects of sin, many reach it only with difficulty, late, or mixed with error. That is one reason God also revealed what reason could in principle know (Dei Filius ch. 2; DH 3005; CCC 37–38).
Where it comes from
Scripture. The Book of Wisdom criticises those who studied the world but did not reach its maker: “from the greatness and beauty of created things comes a corresponding perception of their Creator” (Wisdom 13:5). St Paul goes further. He says that God’s “invisible nature, namely, his eternal power and deity, has been clearly perceived in the things that have been made”, so that those who refuse to honour him “are without excuse” (Romans 1:20). The Psalms say the same in poetry: “The heavens are telling the glory of God” (Psalm 19:1).
When Paul preached to pagans who had never heard of Israel’s God, he began from this natural knowledge. He told the people of Lystra that God “did not leave himself without witness” in the rains and the harvests (Acts 14:17), and the philosophers of Athens that God made us “that they should seek God, in the hope that they might feel after him and find him” (Acts 17:27).
The early Church. St Augustine (died 430) put it in a famous passage. Question the beauty of the earth, the sea and the sky, he wrote: they all answer, “See, we are beautiful.” Their beauty is their confession, and it points to the One who is Beauty and does not change (Sermon 241.2; quoted in CCC 32).
How theologians explain it
From effects to cause. We cannot see God directly. But we can know a cause from its effects, as you can know that someone has been in a room from the footprints they left. St Thomas Aquinas calls this a demonstration quia: reasoning from what is caused back to the cause that must be there (Summa I, q. 2, a. 2). Lesson 2.2 sets out his five arguments of this kind.
Not self-evident, but knowable. Aquinas holds that “God exists” is not self-evident to us, the way “the whole is greater than its part” is. If it were, no one could seriously doubt it. It has to be reached by reasoning (Summa I, q. 2, a. 1).
Two ways of knowing. The manuals distinguish:
| Everyday knowledge | Philosophical knowledge | |
|---|---|---|
| How it works | A quick, natural inference from the order and existence of the world | A careful, step-by-step demonstration |
| Who has it | Most people, at least in an implicit way | Those who study philosophy |
| Example | ”Something must have made all this” | Aquinas’s Five Ways |
Both are real knowledge. The second makes the first precise and answers objections to it.
Errors the Church rejected. The teaching steers between three mistakes:
- Fideism says God can be known only by faith. Then belief would have no reasons at all, and someone without faith could never be shown it is reasonable.
- Agnosticism, in the form given it by Immanuel Kant, says reason cannot reach beyond what we experience, so it can never know anything about God.
- Ontologism says the opposite: we see God directly, and know everything else in him. Then no one could fail to know God, which is plainly false.
Philosophy bridge
The philosophy course’s Natural Theology branch starts from the principle that whatever does not explain its own existence has a cause outside itself. Lesson 2.2 depends on this, and on the idea of a series of causes in which each depends on the one before.
What the Church does not claim
- Not that everyone actually reasons their way to God. The Church teaches that it can be done, not that every person has done it, or done it well.
- Not that those who don’t believe are stupid. Vatican II lists many causes of unbelief, including the failures of believers themselves (Gaudium et Spes 19).
- Not that reason can reach everything about God. Reason can know that God exists and something of what he is. The Trinity and the Incarnation can be known only because God revealed them (Lesson 1.1).
- Not that faith is unnecessary. Knowing that God exists is not yet the faith that saves. Even the demons know God exists (James 2:19).
Check your understanding
- What exactly did Vatican I define about knowing God by reason?
Show answer
That God, the beginning and end of all things, can be known with certainty by the natural light of human reason from the things he has made.
- Give two passages of Scripture that teach that God can be known from creation.
Show answer
Wisdom 13:5 (“from the greatness and beauty of created things comes a corresponding perception of their Creator”) and Romans 1:20. Psalm 19:1 and Acts 14:17 also fit.
- Why does Aquinas say “God exists” is not self-evident to us?
Show answer
Because we do not see God’s essence, so we cannot grasp the truth of the statement just by understanding its terms. If it were self-evident, no one could seriously doubt it. We have to reason to it from God’s effects.
- Explain the difference between fideism and ontologism.
Show answer
Fideism says God can be known only by faith, so reason has no part in it. Ontologism says we see God directly, so knowing him needs no reasoning at all. The Church rejects both: God can be known by reason, but only indirectly, from his effects.
- If God can be known by reason, why did he also need to reveal his existence?
Show answer
Because in practice few people would reach this knowledge, and then only after a long time and mixed with errors. Revelation lets everyone know it easily, firmly and without error.