Here's a more concise (and otherwise improved) version of a worry I was playing about with in my post a few days ago, and a look at a couple of manouevres that might be made in response.
There is a tension between the following three claims:
1. One is committed to Fs iff there are Fs among the range of the quantifiers appearing in one's best theory.
2. According to best theory, it is an ontically vague matter whether an F exists.
3. Someone whose best theory (correctly) asserts that it is a vague matter whether an F exists isn’t committed to saying that an F exists, nor is she committed to saying that one does not exist.
By 3, if 2 is true then we are not committed to the existence of an F. By 1, therefore, no F is among the range of the quantifiers appearing in our best theory. Hence our best theory can also truly contain the sentence ‘Nothing is an F’. But how can it be that our best theory can truly assert both that it is a vague matter whether or not there is an F and that nothing is an F? If nothing is an F, then it is not a vague matter whether or not there is an F: it is settled that there isn’t one.
Short of ditching 1 altogether, there seem to be two possible lines of response which a defender of 2 could appeal to. One is to argue (contra 3) that it is a vague matter whether the defender of vague existence is committed to Fs. The other is to revise 1 so that it reads: one is committed to Fs iff, determinately, there are Fs among the range of the quantifiers appearing in one's best theory.
I don’t find the first response appealing; acknowledging ontic vagueness seems to me to be a paradigmatic way of remaining decidedly non-committal. The second, though, may be thought more promising. Given that the defender of vague existence for Fs is not committed to Fs, the revised version of 1 will only deliver that it is not determinately true that there are Fs among the range of the quantifiers appearing in her best theory. This won’t entail that her best theory can truly say ‘Nothing is an F’.
However, some independent motivation for the revision is needed if it is not to appear ad hoc. And I'm not sure what form that motivation might take. There doesn’t seem (for all that’s been said here) to be anything wrong with the original 1 except that it makes claims of vague existence problematic. But until we are persuaded that vague existence is actually unproblematic, why should we regard that as a reason to revise 1?