Some theistic proofs are brief (Anselm’s Proslogion proof, St. Thomas’s Five Ways, Leibniz’s Monadology proofs, Paley’s watchmaker) and others are much longer (Augustine’s argument in The Free Choice of the Will, Aquinas’s Contra Gentiles proof, Scoutus’s triple point) The first are easier to know, but over time they clutter up with the objections levied against them, so that the proof can become more as a locus of controversies than an argument we can simply read for ourselves. One value of the longer proof is that, though it is harder to puzzle out all the moving parts, the proof itself is not a lightning rod for perpetual controversy. Descartes’s proof is one of the longer ones. Here’s an attempt to tease out the working premises:
1.) To exist as an idea is different from being real.
Descartes calls ideas things with objective existence (i.e. they exist as objects) as opposed to things with real existence, which are said to have formal existence.
2.) Things with objective existence are effects of something with formal existence.
The idea of a unicorn had to come after real horses, or from something else from which we got the idea of white, horse, and horn. In the words of the next premise, objective realities are less real than formal ones.
3.) Things with objective existence arrange themselves in layers of the more and less real.
“More and less real” means this: A is more real than B if the actuality of A is presupposed to the possibility of B. And so on this account, accidents are less real than substances, finite changeable substances are less real than the matter, energy, and physical laws that existed before them, and a Creator is more real than matter, energy, and physical laws.
4.) Causes have at least as much reality as their effects.
True by definition of “more and less real” given above.
Conclusion: The objective existence of my idea of God is caused by God’s actual existence.
While one can explain the objective being of something as merely objective by pointing to any old formal reality, and in this sense we could explain God by positing the humblest formal reality, nevertheless this would not suffice to explain the idea precisely of God, which exists at the highest possible layer of being and therefore is proportionate to some formal reality at that level. We could not have come up with an idea of centaurs from merely considering the various accidents that a centaur might possibly have. One needed more than just “orange” and “two-meters high” to come up with such an idea – we needed experience of various substances and animals. Thus to explain the idea of God as such there must be some formal reality at the divine level of existence, which is, by definition, God.