Full Idea
Another way of saying what objects a theory requires is to say that they are the objects that some of the predicates of the theory have to be true of, in order for the theory to be true.
Gist of Idea
Theories are committed to objects of which some of its predicates must be true
Source
Willard Quine (Existence and Quantification [1966], p.95)
Book Reference
Quine,Willard: 'Ontological Relativity and Other Essays' [Columbia 1969], p.95
A Reaction
The other was for the objects to be needed by the bound variables of the theory. This is the first-order approach, that predication is a commitment to an object. So what of predicates which have no application?