After reading some of Maverick’s other posts on the subject, and reading some material he sent me, it is clear I have misrepresented his argument. Although I am still some way from understanding it, I think it is this.
Suppose there is only one American philosopher, and suppose that it is Vallicella. Then the sentence ‘an American philosopher exists’ is true because Vallicella (qua American philosopher) exists. Now we can translate ‘an American philosopher exists’ into ‘some philosopher is American’, which reduces the verb ‘exists’ to the copula ‘is’. But we can’t translate ‘Vallicella exists’ in the same way. Thus general existential statements presuppose the truth of singular existential statements (or a disjunction or conjunction of singular existential statements). But we cannot analyse away ‘exists’ from singular existential statements. Therefore there is circularity: the same word appears on the right and left hand side of the definition. An American philosopher exists if and only if Vallicella exists.
But there is an obvious route out of this problem. What actually makes ‘some philosopher is American’ true is ‘Vallicella is an American philosopher’, which does not use the word ‘exist’. Vallicella may object that ‘Vallicella exists’ has to be true for that to work. Certainly, but we can reply in two ways. We could suppose that empty proper names are meaningless, and that ‘Vallicella’ is only meaningful because it names something. I.e. if it names something, it must name an existing something. ‘Vallicella exists’ is therefore true in virtue of the meaning of the proper name ‘Vallicella’. Or we could allow that empty proper names are meaningful, and that they have a sense but not a reference. Then we can appeal to the idea of instantiation, as with general concepts. ‘Vallicella exists’ means that the sense of ‘Vallicella’ has a referent or instance. ‘An American philosopher exists’ means that the sense of ‘American philosopher’ has an instance.
That is, either common names and proper names fall into different logical categories, in which case we don’t need to use the word ‘exists’ in singular sentences at all. Or they fall into the same category, in which case we can analyse singular existential statements exactly as we analyse general existential statements. In neither case is the definition of ‘exists’ circular.