The real problem lies elsewhere. Although there is a sense in which according to the criterion a theory that is ontologically committed to Ks must have Ks among the values of its variables, it appears to be the wrong sense. If the interpretation of ‘K’ is not held fixed, then the elements of the domain that are in the extension of ‘K’ have nothing to do with whether Ks exist, as ‘K’ is actually interpreted. It may seem a good thing that ontological commitment to unicorns has nothing to do with unicorns—since there aren't any. But now ontological commitment to elephants also has nothing to do with elephants, and that's wrong. The ontological commitments of a theory should depend on the meanings of its terms, not just on its logical form. Could one instead go for a hybrid criterion, metalinguistic for empty kinds, extensional for non-empty kinds?  (This is suggested by Quine's talk of “retreating to a semantical plane”.) That would only compound the problem. For then we could not know which criterion to apply to a theory prior to knowing what exists, and the criterion could not play a foundational role in ontology. 