Full Idea
How are we to conceive of logical objects? My only answer is, we conceive of them as extensions of concepts or, more generally, as ranges of values of functions ...what other way is there?
Gist of Idea
Logical objects are extensions of concepts, or ranges of values of functions
Source
Gottlob Frege (Letters to Russell [1902], 1902.07.28), quoted by J. Alberto Coffa - The Semantic Tradition from Kant to Carnap 7 epigr
Book Reference
Coffa,J.Alberto: 'The Semantic Tradition from Kant to Carnap' [CUP 1993], p.113
A Reaction
This is the clearest statement I have found of what Frege means by an 'object'. But an extension is a collection of things, so an object is a group treated as a unity, which is generally how we understand a 'set'. Hence Quine's ontology.