Full Idea
Abstract entities (universals) are admitted to an ontology by Quine's criterion if they must be supposed to exist (or subsist) in order to make the propositions of an accepted theory true.
Gist of Idea
Universals are acceptable if they are needed to make an accepted theory true
Source
report of Willard Quine (works [1961]) by Dale Jacquette - Abstract Entity p.3
Book Reference
'Cambridge Dictionary of Philosophy', ed/tr. Audi,Robert [CUP 1995], p.3