Full Idea
Quine doubts the existence of properties, and, trying to be helpful, suggests that variables ranging over properties be replaced with variables ranging over respectable extensional entities like sets, so we can 'identify' a property with a singleton set.
Gist of Idea
Quine suggests that properties can be replaced with extensional entities like sets
Source
report of Willard Quine (works [1961]) by Stewart Shapiro - Higher-Order Logic 2.1
Book Reference
'Blackwell Guide to Philosophical Logic', ed/tr. Goble,Lou [Blackwell 2001], p.36
A Reaction
This strikes me as a classic modern heresy, a slippery slope that loses all grip on what a property is, replacing it with entities that mean nothing, but make the logic work.