For reasons that Bill the Maverick will understand, I have recently been exercised by his argument that the 'thin' definition of existence is circular. He says here, for example, that "to account for the existence of an individual in terms of the instantiation of some concept or property is blatantly circular: if a first-level property is instantiated,then it is instantiated by something that exists" (my emphasis).
The thin conception, as I understand it, is that the first statement below is broadly equivalent to any of the three statements that follow it.
1. An American philosopher exists
2. The concept 'American philosopher' is instantiated
3. There is an American philosopher
4. Some philosopher is American
Maverick, as I understand him, believes that this equivalence involves a circularity. I.e. The definition of the first statement in terms of the second (and probably the third or the fourth, though he has never said this) is circular. What does he mean?
By way of preliminary, let's talk about what a circular definition actually is. A circular definition is one where the left hand term, the term to be defined, the definiendum, contains a term or a word that is also contained in the defining expression, the definiens. For example, if I say, as some people do, that money is whatever people treat as money, that is a circular definition, because the word 'money' is used to define itself. When we look at the right hand of the definition, we have to ask what 'money' means, and so we ask again by putting it on the left, and if we get the same reply, we go on and on in an endless circle.
Now I emphasised 'term' and 'word', because that is very important. The definition is circular because of the repeated word, not because of a repeated concept. Clearly the concepts corresponding to the left and right hand expressions must be repeated, otherwise it wouldn't be a definition. The whole point of a definition is to explain an expression you don't understand in terms of an expression you do understand. For example, you may not understand the word 'mutton', but you may understand the word 'lamb'. So I tell you that mutton is the same thing as lamb, and you understand. Now it's no good objecting that the definition is circular because the concept of mutton presuppposes the concept of lamb, indeed is identical to it. Of course it does, and that is the whole point. I am explaining that the concept corresponding to 'mutton' is identical to the concept corresponding to 'lamb'. So the fact that the same concept occurs (as it were) on both sides of the definition does not mean the definition is circular. On the contrary, that is what makes the definition work in the first place. It's a repeated word that is the only problem.
With that preliminary out of the way, it seems clear that defining (1) above in terms of any of the following three statements is in no way circular. The word 'exists', which is the one we want to explain or define, does not occur in any of the three defining statements. So why does Maverick there is any circularity?
Now he says 'if a first-level property is instantiated, then it is instantiated by something that exists" (my emphasis). That seems to me like objecting that if something is lamb, then it is lamb which is mutton, and so defining mutton as lamb is circular. Which is absurd. Now he may mean that the adjective 'existing' adds something to the expression 'American philosopher', and so 'American philosopher' and 'existing American philosopher' are not equivalent. If that is true, then the definition certainly would be circular. For the statement 'some philosopher is American' of (4) above would be elliptical for 'some philosopher who exists is American', and the defining right hand expression would thus contain the term 'exists', which was on the left hand side. But that is precisely what the thin theorist is denying, indeed strenuously denying. 'Existing philosopher' and 'philosopher' are, for him, exactly the same, and so 'instantiated by something' and 'instantiated by something that exists' are exactly the same, just as 'instantiated by something' and 'instantiated by something that is something' are the same. Indeed, just as 'cooking lamb' and 'cooking lamb which is mutton' are the same.
Or does he mean that the concept of existence is presupposed in statements (2)-(4) above? Well of course it is, just as the concept of mutton is presupposed by the concept of lamb. To understand the statement 'someone is an American philosopher', you have to understand the concept of existence, for the existence of an American philosopher is exactly what the statement asserts. Similarly, in order to understand 'lamb is on the menu' you have to understand the concept of mutton, for the concept of mutton and the concept of lamb are one and the same. But that doesn't mean that the definition 'mutton is lamb' is circular.
Perhaps Maverick intends something different. But if he does, I am very far from understanding it, and the first duty of a philosopher is enable understanding.