Full Idea
The fictionalist can say that the sense in which '2+2=4' is true is pretty much the same as the sense in which 'Oliver Twist lived in London' is true. They are true 'according to a well-known story', or 'according to standard mathematics'.
Gist of Idea
Fictionalists say 2+2=4 is true in the way that 'Oliver Twist lived in London' is true
Source
Hartry Field (Realism, Mathematics and Modality [1989], 1.1.1), quoted by Michèle Friend - Introducing the Philosophy of Mathematics 6.3
Book Reference
Field,Hartry: 'Realism, Mathematics and Modality' [Blackwell 1989], p.3
A Reaction
The roots of this idea are in Carnap. Fictionalism strikes me as brilliant, but poisonous in large doses. Novels can aspire to artistic truth, or to documentary truth. We invent a fiction, and nudge it slowly towards reality.