Full Idea
Abstraction is usually explained in one of four ways: the Way of Example (cf. donkeys and numbers), the Way of Conflation (same as sets), the Negative Way (non-spatial and non-causal) or the Way of Abstraction (incomplete descriptions).
Gist of Idea
Abstraction is usually explained either by example, or conflation, or abstraction, or negatively
Source
David Lewis (On the Plurality of Worlds [1986], 1.7)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.82
A Reaction
[Compressed; a footnote dismisses Dummett's fifth way] Example has blurred boundaries, and explains nothing. Gerrymandered sets don't produce concepts. Negative accounts explain nothing. So it's the Way of Abstraction!