Full Idea
The question to be posed is whether is-a-member-of should be considered a logical constant, that is, does logic include set theory. Frege, Russell and Whitehead held that it did.
Gist of Idea
The logicists held that is-a-member-of is a logical constant, making set theory part of logic
Source
Alex Orenstein (W.V. Quine [2002], Ch.5)
Book Reference
Orenstein,Alex: 'W.V. Quine' [Princeton 2002], p.99
A Reaction
This is obviously the key element in the logicist programme. The objection seems to be that while first-order logic is consistent and complete, set theory is not at all like that, and so is part of a different world.