A: I just can’t make sense of the Thomistic idea that God is not a being, or that he is outside being. It seems like I know what I mean when I say something is or is not, and if God exists, then he is; and if not, then not. How can anyone talk me out of the idea that I have an idea of existence that is common to everything?
B: St. Thomas says things like these because he speaks of God as a cause.
A: Well, causes either are or aren’t, right? I’ve got a box in my head marked “exists” and another marked “doesn’t exist”, and everything has to go in one or the other. This is just the principle of contradiction.
B: So you recognize something common to everything, and that everything must really have/be it or not?
B: We Thomists agree to this, but we think it shows exactly the opposite of what you take it to show.
B: Because if anything is common to many things, then none of the members of that multitude could be the cause of it.
A: Why not? Assume there’s some torch that lights everything else on fire, or someone who knows X and teaches it to everyone else who knows it. What’s so odd about this? Isn’t this how things normally happen?
B: That might be so, but all these examples just help themselves to the thing they want to explain. Sure, given something on fire, or some one that knows X you can explain how something else on fire, or someone else coming to know the thing. It certainly counts as some sort of explanation to simply assume the explanans, but we have to put some pretty heavy qualifications on what we mean by an explanation in order to count this as one.
A: That works for those particular examples, why can’t the class of existent things be like a barber how shaves everyone, including himself, or a doctor who heals everyone, even himself? Here you have a member of the class explaining everyone in the class. God is in the class of existent things because he causes existence in himself along with everyone else. Someone is healed in exactly the same way (say, by taking medicine) regardless of whether they are a doctor or not. So why doesn’t everything exist in exactly the same way too?
B: But a doctor isn’t sick because he’s a doctor, nor does being a barber make one shaggy. The sick have doctors among them only accidentally. No member of the group as such heals themselves. This matters: you can’t learn much of anything about sickness by studying the knowledge that makes someone a doctor; nor can you learn to shave simply by learning all you can about facial hair.
A: So where does that leave us?
B: Saying that you can’t explain something common to many things by pointing to any of the members of the group, unless you’re taking it as given, or you muddle together distinct groups.
A: And so if my idea of existence is really common, then I either have to take God’s existence as a given or be only saying he exists accidentally.
B: Right – except it’s ambiguous to “take existence as given”. What it means here is that you wouldn’t explain existence even of creatures. Remember, this was the first example that came up.