Why only one God?
What reason can say about who God is, with Thomas Aquinas.
The three ways show that there is something first: what changes things without changing, being itself, and what is necessary in itself. But what can reason say about what it is?
Thomas Aquinas starts with what God is not: not composite, not limited, not one among many. Then he asks how our words can say anything about God at all. Read the argument in short first. Then walk through it step by step, and at each step pick the most common objection to see how to answer it.
The argument in short
The argument in five sentences, to learn by heart.
- The first is pure act and being itself. So it has no parts: God is simple.
- What is pure act lacks nothing. God is perfect and good, and good of himself.
- God has not received his being in anything that limits it. So he is infinite.
- What is its own essence cannot exist in several copies. There is only one God.
- Our words about God are true but fall short. They are said of him by analogy, for God is not one being among others.
Concepts you needEssence and existence · Act and potency
Essence and existence essentia, esseEssence is what a thing is. Existence, or being, is that it is. In everything created the two are distinct: existence is not part of what the thing is.
Example
You know what a rose is without knowing whether there is a rose on the table. To know that, you have to look.
In Thomas
Thomas discusses the distinction in De ente et essentia, ch. 3 (ch. 4 in some editions). In Summa theologiae I q.3 a.4 he shows that only in God are essence and being the same.
Act and potency actus, potentiaPotency is the capacity to become or receive something. Act is its actually being so. What receives something is in potency to it, and what is received is its act.
Example
A seed is a tree in potency, a grown tree is a tree in act. Cold water is hot in potency, until someone heats it.
In Thomas
Thomas applies the concepts to existence: what receives its existence is related to it as potency to act (De ente et essentia, ch. 3). In God there is no potency, and so his essence and his being are one (Summa theologiae I q.3 a.4).
Step by step
or tap a box in the map to see the texts behind it.
Test yourself
Think of your answer first. Nothing is saved.
What does it mean that God is simple?
That he is not composed of anything: no parts, no body, and in him what he is and that he exists are the same.
Why is God good of himself, when we are only partly good?
A thing is good insofar as it is perfect. God is pure act and lacks nothing, so he is good by his own essence. We have received our goodness.
In what way is God infinite?
Not in size but in perfection. His being is not received in anything that limits it.
How do you answer someone who says Christians believe in three gods?
Christians believe in one God in three persons. That God is one, reason can show. That he is three persons we know through revelation, and it is not impossible.
What does it mean that we speak of God by analogy?
Our words mean neither exactly the same of God as of us, nor something completely different. They say something true about God, but fall short.