In The Impiety of Deists, etc., Mersenne also gives this theistic argument:
And if there is no God, no independent being, it would be impossible that one exist, and thus our imagination would exceed all the beings of the word: and the being of our thoughts and our fantasies [phantasies] would infinitely exceed all real beings, and what would be imaginary would surpass the true, which cannot be. (p. 75)
There are a couple of interesting things. First, often Leibniz gets credited with noticing that if God possibly exists, then God actually exists. But here we see Mersenne claiming the contrapositive, almost two decades before Descartes' Meditations (in an objection to Descartes' Meditations, Mersenne also makes the point in the Leibniz form).
Second, we get an interesting argument:
- If God doesn't exist, our imagination exceeds reality.
- Our imagination does not exceed reality.
- So, God exists.
When thinking about this argument, I was initially puzzled why Mersenne starts the argument by arguing that if there is no God then the existence of God is impossible. After all, (1)-(3) doesn't seem to require the impossibility of God, just the non-actuality of God. My tentative interpretation is that Mersenne has in mind a fairly strong notion of "exceeds". Possibility has a certain foot in reality, and so for imagination to fully exceed reality, one would have to not only imagine something greater than what actually exists, but greater than what is possible. Now, God is greater than all non-divine possibilities. So if God is impossible, then the content of our thoughts outruns not just actuality but possibility, and that's what makes that content strongly outrun reality.
If this is right, then we can expand the argument as follows:
- If God doesn't exist, it is impossible for God to exist. (Premise)
- God is greater than all possibilities and actualities other than God. (Premise)
- We can think of God. (Premise)
- We cannot think of anything that exceeds all actualities and possibilities.
- God doesn't exist. (Supposition for reductio)
- God is not a possibility or actuality. (4 and 8)
- We can think of something that exceeds all actualities and possibilities. (5, 6 and 9)
- Contradiction! (7 and 10)
- So, God exists. (By reductio)
Finally, it is rather interesting how Mersenne argues for the thesis if God doesn't exist, he can't exist. In the context of another argument, he says:
He isn't a being, as we supposed, he can't exist: since who would make him, and who would give him being [qui luy donneroit estre]? (p. 119)My first thought on this was that Mersenne subscribes to the causal theory of possibility that I've defended. My second thought, however, was that his argument may be broader. The "who would give him being?" rhetorical question may work on any view on which possibility is grounded in actuality given the plausibility that God's possibility couldn't be grounded in anything other than himself, or else he wouldn't truly be an independent being (and notice the focus on independence in the first Mersenne quote).
By the way, while I am relying on my own translations in the above (partly for fun), professional translations can be found here.