I was reading Descartes' Meditations on First Philosophy. Here is his first argument for God:
Effects have causes.
The cause of a certain effect must contain the properties found in that effect, and must thus be equal to or greater than the effect.
For example, a grain of sand cannot cause a boulder, and a finite thing cannot produce an infinite thing.
Ideas are among those things which have causes. The idea of a lion is caused by a real lion. We may conceive of a griffin by combining the ideas we have of eagles and lions.
Then we come to the idea of God, which (recall), must be produced by something equal to or greater than that idea. The only thing equal to or greater than the idea of God is God himself.
If an effect exists, its cause must also exist.
Therefore, God exists.
Or, we might put this formally as:
- A cause must be greater than or equal to its effect.
- If a cause must be greater than or equal to its effect, then God is the cause of the idea of God.
- If God is the cause of the idea of God, then God exists.
- Therefore, God exists.
Descartes' Meditation III, in which he reasons this, can be read here.