Existence elicited nausea from Sartre's Roquentin, but wonder from Bryan Magee:
. . . no matter what it was that existed, it seemed to me extraordinary beyond all wonderment that it should. It was astounding that anything existed at all. Why wasn't there nothing? By all the normal rules of expectation — the least unlikely state of affairs, the most economical solution to all possible problems, the simplest explanation — nothing is what you would have expected there to be. But such was not the case, self-evidently. (Confessions of a Philosopher, p. 13)
What elicited Magee's wonderment was the self-evident sheer existence of things in general: their being as opposed to their nonbeing. How strange that anything at all exists! Now what could a partisan of the thin conception of Being or existence make of Magee's intuition of existence?
Connoisseurs of this arcana will recognize it as pure Frege:
. . . existence is analogous to number. Affirmation of existence is in fact nothing but denial of the number nought. (Gottlob Frege, Foundations of Arithmetic, 65e)
'Cats exist,' then, says that the number of cats is one or more. Equivalently, it says that the concept cat has one or more instances. Existence, as Frege puts it, is "a property of concepts." It is the property of being instantiated. Since individuals, by definition, cannot be instantiated, it follows that existence cannot be predicated of individuals. Despite linguistic appearances, 'exist(s)' is a second-level predicate.
The Fregean approach has a lot to recommend it. For one thing, it allows for a neat solution to the problem of general negative existentials such as 'Unicorns do not exist.' The sentence is true, but it cannot be about unicorns, since if the sentence is true, they do not exist. The sentence is about the concept unicorn and says that this concept is not instantiated.
So much for a sketch of the Fregean variant of the thin conception of Being or existence. The question before us is whether Magee's wonderment at the sheer existence of things could be expressed in terms of the thin conception. Could we plug in 'things' for 'X' in van Inwagen's formula and say that what Magee and so many others have wondered at is the fact that the number of things is 1 or more? Or that the concept existence has instances?
I don't think so. Sometimes I wonder at the seemingly stranger-than-fiction existence of Saguaro cacti. It is plausible to maintain that the object of my wonderment is the instantiation of the natural kind Carnegiea gigantea, equivalently, the fact that some of the things in the world are Saguaros. But when I wonder at the sheer existence of anything at all, I am not wondering at the fact that some of the things in the world have a certain nature or belong to a certain natural kind. For existing things are not a kind of thing, as Aristotle remarked in more than one place. (We shall have to examine the famous argument at Metaphysics 998b22 that Being is not a summum genus, not a genus generalissimum.) I am wondering that there are things at all.
Note the difference between wondering at the fact that there are things at all, and wondering at the fact that there are things of such-and-such a kind. Struck by the existence of Saguaros, I attend to Saguaro-nature and its being instanced. The wonder is directed at general existence or instantiation. But when I am struck by the sheer existence of things in general, I do not attend to any nature, but to that without which no nature could be instantiated: my wonder is directed at singular existence, the existence of the individuals without which no concept or property or kind could be instantiated.
The following argument will serve as summary:
1. If existence were merely the being-instantiated of concepts or properties or natural kinds or cognate items, then my wonder at the sheer existence of things in general would be a wonder at the being-instantiated of some concept or property or natural kind or cognate item.
2. But my wonder at the sheer existence of things in general — at their being as opposed to their nonbeing — is not a wonder at the being-instantiated of some concept or property or natural kind or cognate item. For existing things are not instances of some concept or property or natural kind called 'existence.' There is no such concept or property or natural kind.
Therefore
3. Existence is not merely the being-instantiated of concepts or properties or natural kinds or cognate items.
Therefore
4. The thin conception (at least as described by van Inwagen above) is inadequate. There is more to existence than the thin conception allows.