DSPW = our book, Descartes: Selected Philosophical Writings, edited by Cottingham, Stoothoff and Murdoch.

1. Causal Principle: an effect cannot have more formal or objective reality than it's cause has formal reality (DSPW 91, AT VII 40-41).

2. Ideas have objective reality equal to the formal reality of their content (see Second Replies “arguments arranged in a geometrical fashion,” DSPW 153, AT VII 161).

3. My idea of God is an idea of an infinite, perfect being - that is, omniscient, omnipotent, eternal, infinitely good, … (DSPW 90, AT VII 40).

4. An infinite, perfect being would have infinite formal reality.

5. By (2) and (4), my idea of God has infinite objective reality.

6. I am not such an infinite being. I am certain, for example, that I am not omniscient (DSPW 96, AT VII 49).

7. Therefore, by (1), (5), and (6), I cannot be the cause of my idea of God. I do not have the formal reality to cause an idea with infinite objective reality.

8. By (1) and (5), only a being which actually is infinite could be the cause of my idea of an infinite being. My idea has infinite objective reality, so its cause must have infinite formal reality.

9. By (7) and (8), there must be a being with infinite formal reality, external to me, which causes my idea of God. That being is God.